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

[Xin Li <xin@zytor.com>]

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@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.

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@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?

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