Re: [PATCH v11 12/18] KVM: x86: Enable guest_memfd shared memory for SW-protected VMs

David Hildenbrand <david@xxxxxxxxxx> · Thu, 5 Jun 2025 19:35:14 +0200

On 05.06.25 18:11, Fuad Tabba wrote:
On Thu, 5 Jun 2025 at 16:49, David Hildenbrand <david@xxxxxxxxxx> wrote:

On 05.06.25 17:37, Fuad Tabba wrote:
Define the architecture-specific macro to enable shared memory support
in guest_memfd for relevant software-only VM types, specifically
KVM_X86_DEFAULT_VM and KVM_X86_SW_PROTECTED_VM.

Enable the KVM_GMEM_SHARED_MEM Kconfig option if KVM_SW_PROTECTED_VM is
enabled.

Co-developed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
   arch/x86/include/asm/kvm_host.h | 10 ++++++++++
   arch/x86/kvm/Kconfig            |  1 +
   arch/x86/kvm/x86.c              |  3 ++-
   3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 709cc2a7ba66..ce9ad4cd93c5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2255,8 +2255,18 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,

   #ifdef CONFIG_KVM_GMEM
   #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_shared_mem(kvm)                       \
+     (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) &&                      \
+      ((kvm)->arch.vm_type == KVM_X86_SW_PROTECTED_VM ||             \
+       (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM))
   #else
   #define kvm_arch_supports_gmem(kvm) false
+#define kvm_arch_supports_gmem_shared_mem(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 b37258253543..fdf24b50af9d 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -47,6 +47,7 @@ config KVM_X86
       select KVM_GENERIC_HARDWARE_ENABLING
       select KVM_GENERIC_PRE_FAULT_MEMORY
       select KVM_GENERIC_GMEM_POPULATE if KVM_SW_PROTECTED_VM
+     select KVM_GMEM_SHARED_MEM if KVM_SW_PROTECTED_VM
       select KVM_WERROR if WERROR

Is $subject and this still true, given that it's now also supported for
KVM_X86_DEFAULT_VM?

True, just not the whole truth :)

I guess a better one would be, for Software VMs (remove protected)?

Now I am curious, what is a Hardware VM? :)

--
Cheers,

David / dhildenb








