On Thu, Jul 17, 2025, Fuad Tabba wrote:
> The original name was vague regarding its functionality.
It was intentionally vague/broad so that KVM didn't end up with an explosion of
Kconfigs.
> This Kconfig option specifically enables and gates the kvm_gmem_populate()
> function, which is responsible for populating a GPA range with guest data.
And obviously selects KVM_GENERIC_MEMORY_ATTRIBUTES...
> The new name, KVM_GENERIC_GMEM_POPULATE, describes the purpose of the
> option: to enable generic guest_memfd population mechanisms.
As above, the purpose of KVM_GENERIC_PRIVATE_MEM isn't just to enable
kvm_gmem_populate(). In fact, the Kconfig predates kvm_gmem_populate(). The
main reason KVM_GENERIC_PRIVATE_MEM was added was to avoid having to select the
same set of Kconfigs in every flavor of CoCo-ish VM, i.e. was to avoid what this
patch does.
There was a bit of mis-speculation in that x86 ended up being the only arch that
wants KVM_GENERIC_MEMORY_ATTRIBUTES, so we should simply remedy that. Providing
KVM_PRIVATE_MEM in x86 would also clean up this mess:
select KVM_GMEM if KVM_SW_PROTECTED_VM
select KVM_GENERIC_MEMORY_ATTRIBUTES if KVM_SW_PROTECTED_VM
select KVM_GMEM_SUPPORTS_MMAP if X86_64
Where KVM_GMEM_SUPPORTS_MMAP and thus KVM_GUEST_MEMFD is selected by X86_64.
I.e. X86_64 is subtly *unconditionally* enabling guest_memfd. I have no objection
to always supporting guest_memfd for 64-bit, but it should be obvious, not buried
in a Kconfig config.
More importantly, the above means it's impossible to have KVM_GMEM without
KVM_GMEM_SUPPORTS_MMAP, because arm64 always selects KVM_GMEM_SUPPORTS_MMAP, and
x86 can only select KVM_GMEM when KVM_GMEM_SUPPORTS_MMAP is forced/selected.
Following that trail of breadcrumbs, x86 ends up with another tautology that isn't
captured. kvm_arch_supports_gmem() is true for literally every type of VM. It
isn't true for every #defined VM type, since it's not allowed for KVM_X86_SEV_VM
or KVM_X86_SEV_ES_VM. But those are recent additions that are entirely optional.
I.e. userspace can create SEV and/or SEV-ES VMs using KVM_X86_DEFAULT_VM.
And if we fix that oddity, and follow more breadcrumbs, we arrive at
kvm_arch_supports_gmem_mmap(), where it unnecessarily open codes a check on
KVM_X86_DEFAULT_VM when in fact the real restriction is that guest_memfd mmap()
is currently incompatible with kvm_arch_has_private_mem().
I already have a NAK typed up for patch 3 for completely unrelated reasons (adding
arch.supports_gmem creates unnecessary potential for bugs, e.g. allows checking
kvm_arch_supports_gmem() before the flag is set). That's all the more reason to
kill off as many of these #defines and checks as possible.
Oh, and that also ties into Xiaoyao's question about what to do with mapping
guest_memfd into a memslot without a guest_memfd file descriptor. Once we add
private vs. shared tracking in guest_memfd, kvm_arch_supports_gmem_mmap() becomes
true if CONFIG_KVM_GUEST_MEMFD=y.
Heh, so going through all of that, KVM_PRIVATE_MEM just ends up being this:
config KVM_PRIVATE_MEM
depends on X86_64
select KVM_GENERIC_MEMORY_ATTRIBUTES
bool
which means my initial feedback that prompted this becomes null and void :-)
That said, I think we should take this opportunity to select KVM_GENERIC_MEMORY_ATTRIBUTES
directly instead of having it selected from "config KVM". There's a similar
oddity with TDX.
> improves clarity for developers and ensures the name accurately reflects
> the functionality it controls, especially as guest_memfd support expands
> beyond purely "private" memory scenarios.
>
> Note that the vm type KVM_X86_SW_PROTECTED_VM does not need the populate
> function. Therefore, ensure that the correct configuration is selected
> when KVM_SW_PROTECTED_VM is enabled.
>
> Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
> Reviewed-by: Shivank Garg <shivankg@xxxxxxx>
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> Co-developed-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> ---
> arch/x86/kvm/Kconfig | 7 ++++---
> include/linux/kvm_host.h | 2 +-
> virt/kvm/Kconfig | 2 +-
> virt/kvm/guest_memfd.c | 2 +-
> 4 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 2eeffcec5382..12e723bb76cc 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -46,7 +46,8 @@ config KVM_X86
> select HAVE_KVM_PM_NOTIFIER if PM
> select KVM_GENERIC_HARDWARE_ENABLING
> select KVM_GENERIC_PRE_FAULT_MEMORY
> - select KVM_GENERIC_PRIVATE_MEM if KVM_SW_PROTECTED_VM
> + select KVM_GMEM if KVM_SW_PROTECTED_VM
> + select KVM_GENERIC_MEMORY_ATTRIBUTES if KVM_SW_PROTECTED_VM
> select KVM_WERROR if WERROR
>
> config KVM
> @@ -95,7 +96,7 @@ config KVM_SW_PROTECTED_VM
> config KVM_INTEL
> tristate "KVM for Intel (and compatible) processors support"
> depends on KVM && IA32_FEAT_CTL
> - select KVM_GENERIC_PRIVATE_MEM if INTEL_TDX_HOST
> + select KVM_GENERIC_GMEM_POPULATE if INTEL_TDX_HOST
> select KVM_GENERIC_MEMORY_ATTRIBUTES if INTEL_TDX_HOST
> help
> Provides support for KVM on processors equipped with Intel's VT
> @@ -157,7 +158,7 @@ config KVM_AMD_SEV
> depends on KVM_AMD && X86_64
> depends on CRYPTO_DEV_SP_PSP && !(KVM_AMD=y && CRYPTO_DEV_CCP_DD=m)
> select ARCH_HAS_CC_PLATFORM
> - select KVM_GENERIC_PRIVATE_MEM
> + select KVM_GENERIC_GMEM_POPULATE
> select HAVE_KVM_ARCH_GMEM_PREPARE
> select HAVE_KVM_ARCH_GMEM_INVALIDATE
> help
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 755b09dcafce..359baaae5e9f 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2556,7 +2556,7 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
> int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order);
> #endif
>
> -#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
> +#ifdef CONFIG_KVM_GENERIC_GMEM_POPULATE
> /**
> * kvm_gmem_populate() - Populate/prepare a GPA range with guest data
> *
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 49df4e32bff7..559c93ad90be 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -116,7 +116,7 @@ config KVM_GMEM
> select XARRAY_MULTI
> bool
>
> -config KVM_GENERIC_PRIVATE_MEM
> +config KVM_GENERIC_GMEM_POPULATE
> select KVM_GENERIC_MEMORY_ATTRIBUTES
> select KVM_GMEM
This is where things really start to break down. Selecting KVM_GUEST_MEMFD and
KVM_GENERIC_MEMORY_ATTRIBUTES when KVM_GENERIC_PRIVATE_MEM=y is decent logic.
*Selecting* KVM_GUEST_MEMFD from a sub-feature of guest_memfd is weird.
I don't love HAVE_KVM_ARCH_GMEM_INVALIDATE and HAVE_KVM_ARCH_GMEM_PREPARE, as I
think they're too fine-grained. But that's largely an orthogonal problem, and
it's not clear that bundling them together would be an improvement. So, I think
we should just follow those and add HAVE_KVM_ARCH_GMEM_POPULATE, selected by SEV
and TDX.
The below diff applies on top. I'm guessing there may be some intermediate
ugliness (I haven't mapped out exactly where/how to squash this throughout the
series, and there is feedback relevant to future patches), but IMO this is a much
cleaner resting state (see the diff stats).
---
arch/arm64/include/asm/kvm_host.h | 5 -----
arch/arm64/kvm/Kconfig | 3 +--
arch/x86/include/asm/kvm_host.h | 15 +-------------
arch/x86/kvm/Kconfig | 10 +++++----
arch/x86/kvm/x86.c | 13 ++++++++++--
include/linux/kvm_host.h | 34 +++++--------------------------
virt/kvm/Kconfig | 11 +++-------
virt/kvm/guest_memfd.c | 10 +++++----
virt/kvm/kvm_main.c | 8 +++-----
9 files changed, 36 insertions(+), 73 deletions(-)
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 63f7827cfa1b..3408174ec945 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1674,9 +1674,4 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt);
void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res0, u64 *res1);
void check_feature_map(void);
-#ifdef CONFIG_KVM_GMEM
-#define kvm_arch_supports_gmem(kvm) true
-#define kvm_arch_supports_gmem_mmap(kvm) IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP)
-#endif
-
#endif /* __ARM64_KVM_HOST_H__ */
diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 323b46b7c82f..bff62e75d681 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -37,8 +37,7 @@ menuconfig KVM
select HAVE_KVM_VCPU_RUN_PID_CHANGE
select SCHED_INFO
select GUEST_PERF_EVENTS if PERF_EVENTS
- select KVM_GMEM
- select KVM_GMEM_SUPPORTS_MMAP
+ select KVM_GUEST_MEMFD
help
Support hosting virtualized guest machines.
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e1426adfa93e..d93560769465 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2276,21 +2276,8 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
int tdp_max_root_level, int tdp_huge_page_level);
-#ifdef CONFIG_KVM_GMEM
+#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
#define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
-#define kvm_arch_supports_gmem(kvm) ((kvm)->arch.supports_gmem)
-
-/*
- * CoCo VMs with hardware support that use guest_memfd only for backing private
- * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled.
- */
-#define kvm_arch_supports_gmem_mmap(kvm) \
- (IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP) && \
- (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM)
-#else
-#define kvm_arch_has_private_mem(kvm) false
-#define kvm_arch_supports_gmem(kvm) false
-#define kvm_arch_supports_gmem_mmap(kvm) false
#endif
#define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 2eeffcec5382..afcf8628f615 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -46,8 +46,8 @@ config KVM_X86
select HAVE_KVM_PM_NOTIFIER if PM
select KVM_GENERIC_HARDWARE_ENABLING
select KVM_GENERIC_PRE_FAULT_MEMORY
- select KVM_GENERIC_PRIVATE_MEM if KVM_SW_PROTECTED_VM
select KVM_WERROR if WERROR
+ select KVM_GUEST_MEMFD if X86_64
config KVM
tristate "Kernel-based Virtual Machine (KVM) support"
@@ -84,6 +84,7 @@ config KVM_SW_PROTECTED_VM
bool "Enable support for KVM software-protected VMs"
depends on EXPERT
depends on KVM && X86_64
+ select KVM_GENERIC_MEMORY_ATTRIBUTES
help
Enable support for KVM software-protected VMs. Currently, software-
protected VMs are purely a development and testing vehicle for
@@ -95,8 +96,6 @@ config KVM_SW_PROTECTED_VM
config KVM_INTEL
tristate "KVM for Intel (and compatible) processors support"
depends on KVM && IA32_FEAT_CTL
- select KVM_GENERIC_PRIVATE_MEM if INTEL_TDX_HOST
- select KVM_GENERIC_MEMORY_ATTRIBUTES if INTEL_TDX_HOST
help
Provides support for KVM on processors equipped with Intel's VT
extensions, a.k.a. Virtual Machine Extensions (VMX).
@@ -135,6 +134,8 @@ config KVM_INTEL_TDX
bool "Intel Trust Domain Extensions (TDX) support"
default y
depends on INTEL_TDX_HOST
+ select KVM_GENERIC_MEMORY_ATTRIBUTES
+ select HAVE_KVM_ARCH_GMEM_POPULATE
help
Provides support for launching Intel Trust Domain Extensions (TDX)
confidential VMs on Intel processors.
@@ -157,9 +158,10 @@ config KVM_AMD_SEV
depends on KVM_AMD && X86_64
depends on CRYPTO_DEV_SP_PSP && !(KVM_AMD=y && CRYPTO_DEV_CCP_DD=m)
select ARCH_HAS_CC_PLATFORM
- select KVM_GENERIC_PRIVATE_MEM
+ select KVM_GENERIC_MEMORY_ATTRIBUTES
select HAVE_KVM_ARCH_GMEM_PREPARE
select HAVE_KVM_ARCH_GMEM_INVALIDATE
+ select HAVE_KVM_ARCH_GMEM_POPULATE
help
Provides support for launching encrypted VMs which use Secure
Encrypted Virtualization (SEV), Secure Encrypted Virtualization with
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ca99187a566e..b6961b4b7aee 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12781,8 +12781,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
kvm->arch.vm_type = type;
kvm->arch.has_private_mem = (type == KVM_X86_SW_PROTECTED_VM);
- kvm->arch.supports_gmem =
- type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
/* Decided by the vendor code for other VM types. */
kvm->arch.pre_fault_allowed =
type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
@@ -13708,6 +13706,16 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_arch_no_poll);
+#ifdef CONFIG_KVM_GUEST_MEMFD
+/*
+ * KVM doesn't yet support mmap() on guest_memfd for VMs with private memory
+ * (the private vs. shared tracking needs to be moved into guest_memfd).
+ */
+bool kvm_arch_supports_gmem_mmap(struct kvm *kvm)
+{
+ return !kvm_arch_has_private_mem(kvm);
+}
+
#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order)
{
@@ -13721,6 +13729,7 @@ void kvm_arch_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end)
kvm_x86_call(gmem_invalidate)(start, end);
}
#endif
+#endif
int kvm_spec_ctrl_test_value(u64 value)
{
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2c1dcd3967d9..a9f31b2b63b1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -719,39 +719,15 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
}
#endif
-/*
- * Arch code must define kvm_arch_has_private_mem if support for guest_memfd is
- * enabled.
- */
-#if !defined(kvm_arch_has_private_mem) && !IS_ENABLED(CONFIG_KVM_GMEM)
+#ifndef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
{
return false;
}
#endif
-/*
- * Arch code must define kvm_arch_supports_gmem if support for guest_memfd is
- * enabled.
- */
-#if !defined(kvm_arch_supports_gmem) && !IS_ENABLED(CONFIG_KVM_GMEM)
-static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
-{
- return false;
-}
-#endif
-
-/*
- * Returns true if this VM supports mmap() in guest_memfd.
- *
- * Arch code must define kvm_arch_supports_gmem_mmap if support for guest_memfd
- * is enabled.
- */
-#if !defined(kvm_arch_supports_gmem_mmap)
-static inline bool kvm_arch_supports_gmem_mmap(struct kvm *kvm)
-{
- return false;
-}
+#ifdef CONFIG_KVM_GUEST_MEMFD
+bool kvm_arch_supports_gmem_mmap(struct kvm *kvm);
#endif
#ifndef kvm_arch_has_readonly_mem
@@ -2539,7 +2515,7 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
static inline bool kvm_memslot_is_gmem_only(const struct kvm_memory_slot *slot)
{
- if (!IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP))
+ if (!IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
return false;
return slot->flags & KVM_MEMSLOT_GMEM_ONLY;
@@ -2596,7 +2572,7 @@ static inline int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot,
int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order);
#endif
-#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
+#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
/**
* kvm_gmem_populate() - Populate/prepare a GPA range with guest data
*
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 96cf4ab0d534..9d472f46ebf1 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -112,15 +112,10 @@ config KVM_GENERIC_MEMORY_ATTRIBUTES
depends on KVM_GENERIC_MMU_NOTIFIER
bool
-config KVM_GMEM
+config KVM_GUEST_MEMFD
select XARRAY_MULTI
bool
-config KVM_GENERIC_PRIVATE_MEM
- select KVM_GENERIC_MEMORY_ATTRIBUTES
- select KVM_GMEM
- bool
-
config HAVE_KVM_ARCH_GMEM_PREPARE
bool
depends on KVM_GMEM
@@ -129,6 +124,6 @@ config HAVE_KVM_ARCH_GMEM_INVALIDATE
bool
depends on KVM_GMEM
-config KVM_GMEM_SUPPORTS_MMAP
- select KVM_GMEM
+config HAVE_KVM_ARCH_GMEM_POPULATE
bool
+ depends on KVM_GMEM
\ No newline at end of file
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index d01bd7a2c2bd..57db0041047a 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -316,9 +316,6 @@ static bool kvm_gmem_supports_mmap(struct inode *inode)
{
const u64 flags = (u64)inode->i_private;
- if (!IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP))
- return false;
-
return flags & GUEST_MEMFD_FLAG_MMAP;
}
@@ -527,6 +524,11 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
return err;
}
+bool __weak kvm_arch_supports_gmem_mmap(struct kvm *kvm)
+{
+ return true;
+}
+
int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
{
loff_t size = args->size;
@@ -730,7 +732,7 @@ int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot, gfn_t gfn)
}
EXPORT_SYMBOL_GPL(kvm_gmem_mapping_order);
-#ifdef CONFIG_KVM_GENERIC_GMEM_POPULATE
+#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long npages,
kvm_gmem_populate_cb post_populate, void *opaque)
{
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f1ac872e01e9..1b609e35303f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1588,7 +1588,7 @@ static int check_memory_region_flags(struct kvm *kvm,
{
u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
- if (kvm_arch_supports_gmem(kvm))
+ if (IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
valid_flags |= KVM_MEM_GUEST_MEMFD;
/* Dirty logging private memory is not currently supported. */
@@ -4915,10 +4915,8 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
#endif
#ifdef CONFIG_KVM_GMEM
case KVM_CAP_GUEST_MEMFD:
- return !kvm || kvm_arch_supports_gmem(kvm);
-#endif
-#ifdef CONFIG_KVM_GMEM_SUPPORTS_MMAP
- case KVM_CAP_GMEM_MMAP:
+ return 1;
+ case KVM_CAP_GUEST_MEMFD_MMAP:
return !kvm || kvm_arch_supports_gmem_mmap(kvm);
#endif
default:
base-commit: 9eba3a9ac9cd5922da7f6e966c01190f909ed640
--
