[PATCH v1 1/1] KVM: x86: Advertise support for LKGS

[PATCH v1 1/1] KVM: x86: Advertise support for LKGS

[Xin Li (Intel)]

Advertise support for LKGS (load into IA32_KERNEL_GS_BASE) to userspace
if the instruction is supported by the underlying CPU.

LKGS is introduced with FRED to completely eliminate the need to swapgs
explicilty.  It behaves like the MOV to GS instruction except that it
loads the base address into the IA32_KERNEL_GS_BASE MSR instead of the
GS segment’s descriptor cache, which is exactly what Linux kernel does
to load a user level GS base.  Thus there is no need to SWAPGS away
from the kernel GS base.

LKGS is an independent CPU feature that works correctly in a KVM guest
without requiring explicit enablement.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---
 arch/x86/kvm/cpuid.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b2d006756e02..bf4a43bd0a47 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -979,6 +979,7 @@ void kvm_set_cpu_caps(void)
 		F(FSRS),
 		F(FSRC),
 		F(WRMSRNS),
+		X86_64_F(LKGS),
 		F(AMX_FP16),
 		F(AVX_IFMA),
 		F(LAM),

base-commit: 05186d7a8e5b8a31be93728089c9c3fee4dab0a2
-- 
2.50.0

Re: [PATCH v1 1/1] KVM: x86: Advertise support for LKGS

[Sean Christopherson]

On Thu, 26 Jun 2025 10:35:21 -0700, Xin Li (Intel) wrote:
> Advertise support for LKGS (load into IA32_KERNEL_GS_BASE) to userspace
> if the instruction is supported by the underlying CPU.
> 
> LKGS is introduced with FRED to completely eliminate the need to swapgs
> explicilty.  It behaves like the MOV to GS instruction except that it
> loads the base address into the IA32_KERNEL_GS_BASE MSR instead of the
> GS segment’s descriptor cache, which is exactly what Linux kernel does
> to load a user level GS base.  Thus there is no need to SWAPGS away
> from the kernel GS base.
> 
> [...]

Applied to kvm-x86 misc, thanks!

[1/1] KVM: x86: Advertise support for LKGS
      https://github.com/kvm-x86/linux/commit/e88cfd50b606

--
https://github.com/kvm-x86/linux/tree/next