linux-kernel - Re: [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features that ignore host kernel support

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Message-ID: <Zoxp4ahfifWA-P34@google.com>
Date: Mon, 8 Jul 2024 15:36:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Hou Wenlong <houwenlong.hwl@...group.com>, 
	Kechen Lu <kechenl@...dia.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features
 that ignore host kernel support

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> > +/*
> > + * Raw Feature - For features that KVM supports based purely on raw host CPUID,
> > + * i.e. that KVM virtualizes even if the host kernel doesn't use the feature.
> > + * Simply force set the feature in KVM's capabilities, raw CPUID support will
> > + * be factored in by kvm_cpu_cap_mask().
> > + */
> > +#define RAW_F(name)						\
> > +({								\
> > +	kvm_cpu_cap_set(X86_FEATURE_##name);			\
> > +	F(name);						\
> > +})
> > +
> >  /*
> >   * Magic value used by KVM when querying userspace-provided CPUID entries and
> >   * doesn't care about the CPIUD index because the index of the function in
> > @@ -682,15 +694,12 @@ void kvm_set_cpu_caps(void)
> >  		F(AVX512VL));
> >  
> >  	kvm_cpu_cap_mask(CPUID_7_ECX,
> > -		F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
> > +		F(AVX512VBMI) | RAW_F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
> >  		F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
> >  		F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
> >  		F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
> >  		F(SGX_LC) | F(BUS_LOCK_DETECT)
> >  	);
> > -	/* Set LA57 based on hardware capability. */
> > -	if (cpuid_ecx(7) & F(LA57))
> > -		kvm_cpu_cap_set(X86_FEATURE_LA57);
> >  
> >  	/*
> >  	 * PKU not yet implemented for shadow paging and requires OSPKE
> 
> Putting a function call into a macro which evaluates into a bitmask is somewhat misleading,
> but let it be...

And weird.  Rather than abuse kvm_cpu_cap_set(), what about adding another variable
scoped to kvm_cpu_cap_init()?

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0e64a6332052..b8bc8713a0ec 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -87,12 +87,10 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
 /*
  * Raw Feature - For features that KVM supports based purely on raw host CPUID,
  * i.e. that KVM virtualizes even if the host kernel doesn't use the feature.
- * Simply force set the feature in KVM's capabilities, raw CPUID support will
- * be factored in by __kvm_cpu_cap_mask().
  */
 #define RAW_F(name)                                            \
 ({                                                             \
-       kvm_cpu_cap_set(X86_FEATURE_##name);                    \
+       kvm_cpu_cap_passthrough |= F(name);                     \
        F(name);                                                \
 })
 
@@ -737,6 +735,7 @@ do {                                                                        \
 do {                                                                   \
        const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);    \
        const u32 __maybe_unused kvm_cpu_cap_init_in_progress = leaf;   \
+       u32 kvm_cpu_cap_passthrough = 0;                                \
        u32 kvm_cpu_cap_emulated = 0;                                   \
        u32 kvm_cpu_cap_synthesized = 0;                                \
                                                                        \
@@ -745,6 +744,7 @@ do {                                                                        \
        else                                                            \
                kvm_cpu_caps[leaf] = (mask);                            \
                                                                        \
+       kvm_cpu_caps[leaf] |= kvm_cpu_cap_passthrough;                  \
        kvm_cpu_caps[leaf] &= (raw_cpuid_get(cpuid) |                   \
                               kvm_cpu_cap_synthesized);                \
        kvm_cpu_caps[leaf] |= kvm_cpu_cap_emulated;                     \