diff mbox series

[v4,15/19] KVM: x86: Allow FRED/LKGS to be advertised to guests

Message ID 20250328171205.2029296-16-xin@zytor.com (mailing list archive)
State New
Headers show
Series Enable FRED with KVM VMX | expand

Commit Message

Xin Li (Intel) March 28, 2025, 5:12 p.m. UTC 
From: Xin Li <xin3.li@intel.com>

Allow FRED/LKGS to be advertised to guests after changes required to
enable FRED in a KVM guest are in place.

LKGS is introduced with FRED to completely eliminate the need to swapgs
explicilty, because

1) FRED transitions ensure that an operating system can always operate
   with its own GS base address.

2) LKGS 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 and an execution of SWAPGS causes #UD
   if FRED transitions are enabled.

A FRED CPU must enumerate LKGS.  When LKGS is not available, FRED must
not be enabled.

Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
---
 arch/x86/kvm/cpuid.c | 2 ++
 1 file changed, 2 insertions(+)
diff mbox series

Patch

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 5e4d4934c0d3..8f290273aee1 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -992,6 +992,8 @@  void kvm_set_cpu_caps(void)
 		F(FZRM),
 		F(FSRS),
 		F(FSRC),
+		F(FRED),
+		F(LKGS),
 		F(AMX_FP16),
 		F(AVX_IFMA),
 		F(LAM),