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

[Sean Christopherson <seanjc@google.com>]

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@intel.com>
> 
> 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@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 --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
>