The shortlog (and changelog intro) are wrong. KVM isn't allowing FRED/LKGS to be advertised to the guest. Userspace can advertise whatever it wants. The guest will break badly without KVM support, but that doesn't stop userspace from advertising a bogus vCPU model. KVM: x86: Advertise support for FRED/LKGS to userspace On Fri, Mar 28, 2025, Xin Li (Intel) wrote: > From: Xin Li <xin3.li@xxxxxxxxx> > > Allow FRED/LKGS to be advertised to guests after changes required to Please explain what LKGS is early in the changelog. I assumed it was a feature of sorts; turns out it's a new instruction. Actually, why wait this long to enumerate support for LKGS? I.e. why not have a patch at the head of the series to enumerate support for LKGS? IIUC, LKGS doesn't depend on FRED. > 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@xxxxxxxxx> > Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx> > Tested-by: Shan Kang <shan.kang@xxxxxxxxx> > --- > arch/x86/kvm/cpuid.c | 2 ++ > 1 file changed, 2 insertions(+) > > 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), These need to be X86_64_F, no? > F(AMX_FP16), > F(AVX_IFMA), > F(LAM), > -- > 2.48.1 >
