Re: [PATCH] KVM: x86: synthesize TSA CPUID bits via SCATTERED_F()

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 09, 2026, Carlos López wrote:
> Hi,
> 
> On 2/9/26 6:48 AM, Jim Mattson wrote:
> > On Sun, Feb 8, 2026 at 1:14 PM Borislav Petkov <bp@alien8.de> wrote:
> >>
> >> On Sun, Feb 08, 2026 at 12:50:18PM -0800, Jim Mattson wrote:
> >>>> /*
> >>>>  * Synthesized Feature - For features that are synthesized into boot_cpu_data,
> >>>>  * i.e. may not be present in the raw CPUID, but can still be advertised to
> >>>>  * userspace.  Primarily used for mitigation related feature flags.
> >>>>                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >>>>  */
> >>>> #define SYNTHESIZED_F(name)
> >>>>
> >>>>> +             SCATTERED_F(TSA_SQ_NO),
> >>>>> +             SCATTERED_F(TSA_L1_NO),
> >>>>
> >>>> And scattered are of the same type.
> >>>>
> >>>> Sean, what's the subtle difference here?
> >>>
> >>> SYNTHESIZED_F() sets the bit unconditionally. SCATTERED_F() propagates
> >>> the bit (if set) from the host's cpufeature flags.

As noted below, SYNTHESIZED_F() isn't entirely unconditional.  kvm_cpu_cap_init()
factors in boot_cpu_data.x86_capability, the problem here is that SYNTHESIZED_F()
is now being used for KVM-only leafs, and so the common code doesn't work as
intended.

> >> Yah, and I was hinting at the scarce documentation.

Maybe I could add a table showing how the XXX_F() macros map to various controls?

> >> SYNTHESIZED_F() is "Primarily used for mitigation related feature flags."
> >> SCATTERED_F() is "For features that are scattered by cpufeatures.h."
> > 
> > Ugh. I have to rescind my Reviewed-by. IIUC, SCATTERED_F() implies a
> > logical and with hardware CPUID, which means that the current proposal
> > will never set the ITS_NO bits.
> 
> Right, I see what you mean now. SCATTERED_F() will set kvm_cpu_caps
> correctly, but then this will clear the bits, because
> kvm_cpu_cap_synthesized is now 0:
> 
>     kvm_cpu_caps[leaf] &= (raw_cpuid_get(cpuid) |
>         kvm_cpu_cap_synthesized);
> 
> So to me it seems like SYNTHESIZED_F() is just wrong,

It was right when I wrote it :-) 

> since it always enables bits for KVM-only leafs.

Yes, I didn't anticipate synthesizing flags into KVM-only leafs.  

> So how about the following
> (I think Binbin Wu suggests this in his other email):
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 819c176e02ff..5e863e213f54 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -769,7 +769,8 @@ do {                                                                        \
>   */
>  #define SYNTHESIZED_F(name)                                    \
>  ({                                                             \
> -       kvm_cpu_cap_synthesized |= feature_bit(name);           \
> +       if (boot_cpu_has(X86_FEATURE_##name))                   \
> +               kvm_cpu_cap_synthesized |= feature_bit(name);           \

I would rather do this:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 88a5426674a1..5f41924987c7 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -770,7 +770,10 @@ do {                                                                       \
 #define SYNTHESIZED_F(name)                                    \
 ({                                                             \
        kvm_cpu_cap_synthesized |= feature_bit(name);           \
-       F(name);                                                \
+                                                               \
+       BUILD_BUG_ON(X86_FEATURE_##name >= MAX_CPU_FEATURES);   \
+       if (boot_cpu_has(X86_FEATURE_##name))                   \
+               F(name);                                        \
 })
 
 /*

because I'd like to keep kvm_cpu_cap_synthesized unconditional and have
kvm_cpu_cap_features reflect what is supported.  And with

  Fixes: 31272abd5974 ("KVM: SVM: Advertise TSA CPUID bits to guests")

because everything was fine before that commit (though it was set up to fail).

>         F(name);                                                \
>  })
>