On 30.04.25 18:56, Fuad Tabba wrote:
Add support for mmap() and fault() for guest_memfd backed memory
in the host for VMs that support in-place conversion between
shared and private. To that end, this patch adds the ability to
check whether the VM type supports in-place conversion, and only
allows mapping its memory if that's the case.
This patch introduces the configuration option KVM_GMEM_SHARED_MEM,
which enables support for in-place shared memory.
It also introduces the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
indicates that the host can create VMs that support shared memory.
Supporting shared memory implies that memory can be mapped when shared
with the host.
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
include/linux/kvm_host.h | 15 ++++++-
include/uapi/linux/kvm.h | 1 +
virt/kvm/Kconfig | 5 +++
virt/kvm/guest_memfd.c | 92 ++++++++++++++++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 4 ++
5 files changed, 116 insertions(+), 1 deletion(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9419fb99f7c2..f3af6bff3232 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -729,6 +729,17 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
}
#endif
+/*
+ * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
+ * private memory is enabled and it supports in-place shared/private conversion.
+ */
+#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)
+static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
+{
+ return false;
+}
+#endif
+
#ifndef kvm_arch_has_readonly_mem
static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
{
@@ -2516,7 +2527,9 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
static inline bool kvm_mem_from_gmem(struct kvm *kvm, gfn_t gfn)
{
- /* For now, only private memory gets consumed from guest_memfd. */
+ if (kvm_arch_gmem_supports_shared_mem(kvm))
+ return true;
After our discussion yesterday, am I correct that we will not be
querying the KVM capability, but instead the "SHARED_TRACKING" (or
however that flag is called) on the underlying guest_memfd instead?
I assume the function would then look something like
if (!kvm_supports_gmem(kvm))
return false;
if (kvm_arch_gmem_supports_shared_mem(kvm))
return .. TBD, test the gmem flag for the slot via gfn
return kvm_mem_is_private(kvm, gfn);
+
return kvm_mem_is_private(kvm, gfn);
}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6ae8ad8934b..8bc8046c7f3a 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -930,6 +930,7 @@ struct kvm_enable_cap {
#define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
#define KVM_CAP_X86_GUEST_MODE 238
#define KVM_CAP_ARM_WRITABLE_IMP_ID_REGS 239
+#define KVM_CAP_GMEM_SHARED_MEM 240
struct kvm_irq_routing_irqchip {
__u32 irqchip;
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 559c93ad90be..f4e469a62a60 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -128,3 +128,8 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
config HAVE_KVM_ARCH_GMEM_INVALIDATE
bool
depends on KVM_GMEM
+
+config KVM_GMEM_SHARED_MEM
+ select KVM_GMEM
+ bool
+ prompt "Enables in-place shared memory for guest_memfd"
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 6db515833f61..8bc8fc991d58 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -312,7 +312,99 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
return gfn - slot->base_gfn + slot->gmem.pgoff;
}
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+/*
+ * Returns true if the folio is shared with the host and the guest.
+ */
+static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
+{
+ struct kvm_gmem *gmem = file->private_data;
+
+ /* For now, VMs that support shared memory share all their memory. */
+ return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
Similar here: likely we want to check the guest_memfd flag.
--
Cheers,
David / dhildenb
