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

[Xin Li]

On 6/24/2025 9:38 AM, Sean Christopherson wrote:
> 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.

I will send LKGS as a separate patch, thus if you prefer you can take
it before the KVM FRED patch set.

> > 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?

Yes.  Both LKGS and FRED are 64-bit only features.

However I assume KVM is 64-bit only now, so X86_64_F is essentially F,
right?

Thanks!
    Xin