Re: [PATCH v7 1/7] KVM: CPUID: Fix IA32_XSS support in CPUID(0xd,i) enumeration

[Yang Weijiang]

On Wed, Oct 02, 2019 at 10:26:10AM -0700, Jim Mattson wrote:
> On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@xxxxxxxxx> wrote:
> >
> > The control bits in IA32_XSS MSR are being used for new features,
> > but current CPUID(0xd,i) enumeration code doesn't support them, so
> > fix existing code first.
> >
> > The supervisor states in IA32_XSS haven't been used in public
> > KVM code, so set KVM_SUPPORTED_XSS to 0 now, anyone who's developing
> > IA32_XSS related feature may expand the macro to add the CPUID support,
> > otherwise, CPUID(0xd,i>1) always reports 0 of the subleaf to guest.
> >
> > Extracted old code into a new filter and keep it same flavor as others.
> >
> > This patch passed selftest on a few Intel platforms.
> >
> > Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  1 +
> >  arch/x86/kvm/cpuid.c            | 94 +++++++++++++++++++++------------
> >  arch/x86/kvm/svm.c              |  7 +++
> >  arch/x86/kvm/vmx/vmx.c          |  6 +++
> >  arch/x86/kvm/x86.h              |  7 +++
> >  5 files changed, 82 insertions(+), 33 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 74e88e5edd9c..d018df8c5f32 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1209,6 +1209,7 @@ struct kvm_x86_ops {
> >         uint16_t (*nested_get_evmcs_version)(struct kvm_vcpu *vcpu);
> >
> >         bool (*need_emulation_on_page_fault)(struct kvm_vcpu *vcpu);
> > +       u64 (*supported_xss)(void);
> >  };
> >
> >  struct kvm_arch_async_pf {
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 22c2720cd948..9d282fec0a62 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -62,6 +62,11 @@ u64 kvm_supported_xcr0(void)
> >         return xcr0;
> >  }
> >
> > +u64 kvm_supported_xss(void)
> > +{
> > +       return KVM_SUPPORTED_XSS & kvm_x86_ops->supported_xss();
> > +}
> > +
> >  #define F(x) bit(X86_FEATURE_##x)
> >
> >  int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> > @@ -414,6 +419,50 @@ static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry, int index)
> >         }
> >  }
> >
> > +static inline void do_cpuid_0xd_mask(struct kvm_cpuid_entry2 *entry, int index)
> > +{
> > +       unsigned int f_xsaves = kvm_x86_ops->xsaves_supported() ? F(XSAVES) : 0;
> 
> Does Intel have CPUs that support XSAVES but don't support the "enable
> XSAVES/XRSTORS" VM-execution control? If so, what is the behavior of
> XSAVESXRSTORS on those CPUs in VMX non-root mode? If not, why is this
> conditional F(XSAVES) here?
Thanks Jim for raising the question.
>From SDM 25.3, if XSAVES/XRSTORS" VM-execution control == 0, these
instructions cause #UD.
> 
> > +       /* cpuid 0xD.1.eax */
> > +       const u32 kvm_cpuid_D_1_eax_x86_features =
> > +               F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | f_xsaves;
> > +       u64 u_supported = kvm_supported_xcr0();
> > +       u64 s_supported = kvm_supported_xss();
> > +       u64 supported;
> > +
> > +       switch (index) {
> > +       case 0:
> > +               entry->eax &= u_supported;
> > +               entry->ebx = xstate_required_size(u_supported, false);
> 
> EBX could actually be zero, couldn't it? Since this output is
> context-dependent, I'm not sure how to interpret it when returned from
> KVM_GET_SUPPORTED_CPUID.
I did't get your concern, What's context_dependent? IIUC, EBX actually
cannot be 0 since x87 and SSE states are always there.
> 
> > +               entry->ecx = entry->ebx;
> > +               entry->edx = 0;
> 
> Shouldn't this be: entry->edx &= u_supported >> 32?
Yes, but the SDM makes me puzzled, it says at the tail: Bits 31 - 00: Reserved,
Maybe I need to follow your suggestion in next version, thanks!
> > +               break;
> > +       case 1:
> > +               supported = u_supported | s_supported;
> > +               entry->eax &= kvm_cpuid_D_1_eax_x86_features;
> > +               cpuid_mask(&entry->eax, CPUID_D_1_EAX);
> > +               entry->ebx = 0;
> > +               entry->edx = 0;
> 
> Shouldn't this be: entry->edx &= s_supported >> 32?
> 
The same as above.
> > +               entry->ecx &= s_supported;
> > +               if (entry->eax & (F(XSAVES) | F(XSAVEC)))
> > +                       entry->ebx = xstate_required_size(supported, true);
> 
> As above, can't EBX just be zero, since it's context-dependent? What
> is the context when processing KVM_GET_SUPPORTED_CPUID? And why do we
> only fill this in when XSAVES or XSAVEC is supported?
> 
IIUC, EBX here reports the states(XCRO | IA32_XSS) in compacted format.
so it's only available when F(XSAVES) or F(XSAVEC) is on.
> > +               break;
> > +       default:
> > +               supported = (entry->ecx & 1) ? s_supported : u_supported;
> > +               if (!(supported & ((u64)1 << index))) {
> 
> Nit: 1ULL << index.
right, thank you.
> 
> > +                       entry->eax = 0;
> > +                       entry->ebx = 0;
> > +                       entry->ecx = 0;
> > +                       entry->edx = 0;
> > +                       return;
> > +               }
> > +               if (entry->ecx)
> > +                       entry->ebx = 0;
> 
> This seems to back up my claims above regarding the EBX output for
> cases 0 and 1, but aside from those subleaves, is this correct? For
> subleaves > 1, ECX bit 1 can be set for extended state components that
> need to be cache-line aligned. Such components could map to a valid
> bit in XCR0 and have a non-zero offset from the beginning of the
> non-compacted XSAVE area.
> 
thank you, should check bit 0 of ecx before clear ebx.
> > +               entry->edx = 0;
> 
> This seems too aggressive. See my comments above regarding EDX outputs
> for cases 0 and 1.
> 
OK, I'll change it together with other parts.
> > +               break;
> > +       }
> > +}
> > +
> >  static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >                                   int *nent, int maxnent)
> >  {
> > @@ -428,7 +477,6 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >         unsigned f_lm = 0;
> >  #endif
> >         unsigned f_rdtscp = kvm_x86_ops->rdtscp_supported() ? F(RDTSCP) : 0;
> > -       unsigned f_xsaves = kvm_x86_ops->xsaves_supported() ? F(XSAVES) : 0;
> >         unsigned f_intel_pt = kvm_x86_ops->pt_supported() ? F(INTEL_PT) : 0;
> >
> >         /* cpuid 1.edx */
> > @@ -482,10 +530,6 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >                 F(ACE2) | F(ACE2_EN) | F(PHE) | F(PHE_EN) |
> >                 F(PMM) | F(PMM_EN);
> >
> > -       /* cpuid 0xD.1.eax */
> > -       const u32 kvm_cpuid_D_1_eax_x86_features =
> > -               F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | f_xsaves;
> > -
> >         /* all calls to cpuid_count() should be made on the same cpu */
> >         get_cpu();
> >
> > @@ -622,38 +666,22 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >                 break;
> >         }
> >         case 0xd: {
> > -               int idx, i;
> > -               u64 supported = kvm_supported_xcr0();
> > -
> > -               entry->eax &= supported;
> > -               entry->ebx = xstate_required_size(supported, false);
> > -               entry->ecx = entry->ebx;
> > -               entry->edx &= supported >> 32;
> > -               if (!supported)
> > -                       break;
> > +               int i, idx;
> >
> > -               for (idx = 1, i = 1; idx < 64; ++idx) {
> > -                       u64 mask = ((u64)1 << idx);
> > +               do_cpuid_0xd_mask(&entry[0], 0);
> > +               for (i = 1, idx = 1; idx < 64; ++idx) {
> >                         if (*nent >= maxnent)
> >                                 goto out;
> > -
> >                         do_host_cpuid(&entry[i], function, idx);
> > -                       if (idx == 1) {
> > -                               entry[i].eax &= kvm_cpuid_D_1_eax_x86_features;
> > -                               cpuid_mask(&entry[i].eax, CPUID_D_1_EAX);
> > -                               entry[i].ebx = 0;
> > -                               if (entry[i].eax & (F(XSAVES)|F(XSAVEC)))
> > -                                       entry[i].ebx =
> > -                                               xstate_required_size(supported,
> > -                                                                    true);
> > -                       } else {
> > -                               if (entry[i].eax == 0 || !(supported & mask))
> > -                                       continue;
> > -                               if (WARN_ON_ONCE(entry[i].ecx & 1))
> > -                                       continue;
> > -                       }
> > -                       entry[i].ecx = 0;
> > -                       entry[i].edx = 0;
> > +                       if (entry[i].eax == 0 && entry[i].ebx == 0 &&
> > +                           entry[i].ecx == 0 && entry[i].edx == 0)
> > +                               continue;
> > +
> > +                       do_cpuid_0xd_mask(&entry[i], idx);
> > +                       if (entry[i].eax == 0 && entry[i].ebx == 0 &&
> > +                           entry[i].ecx == 0 && entry[i].edx == 0)
> > +                               continue;
> > +
> >                         ++*nent;
> >                         ++i;
> >                 }
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index e0368076a1ef..be967bf9a81d 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -7193,6 +7193,11 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
> >         return false;
> >  }
> >
> > +static u64 svm_supported_xss(void)
> > +{
> > +       return 0;
> > +}
> > +
> >  static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
> >         .cpu_has_kvm_support = has_svm,
> >         .disabled_by_bios = is_disabled,
> > @@ -7329,6 +7334,8 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
> >         .nested_get_evmcs_version = NULL,
> >
> >         .need_emulation_on_page_fault = svm_need_emulation_on_page_fault,
> > +
> > +       .supported_xss = svm_supported_xss,
> >  };
> >
> >  static int __init svm_init(void)
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index c6f6b05004d9..a84198cff397 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1651,6 +1651,11 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
> >         return !(val & ~valid_bits);
> >  }
> >
> > +static inline u64 vmx_supported_xss(void)
> > +{
> > +       return host_xss;
> > +}
> 
> Do you really need vendor-specific code for this? Can't you just hoist
> host_xss into common code (x86.c) and use that? [Note that Aaron Lewis
> is currently working on a series that will include that hoisting, if
> you want to wait.]
> 
> 
> >  static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
> >  {
> >         switch (msr->index) {
> > @@ -7799,6 +7804,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
> >         .nested_enable_evmcs = NULL,
> >         .nested_get_evmcs_version = NULL,
> >         .need_emulation_on_page_fault = vmx_need_emulation_on_page_fault,
> > +       .supported_xss = vmx_supported_xss,
> >  };
> >
> >  static void vmx_cleanup_l1d_flush(void)
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index 6594020c0691..fbffabad0370 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -293,6 +293,13 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, unsigned long cr2,
> >                                 | XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
> >                                 | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> >                                 | XFEATURE_MASK_PKRU)
> > +
> > +/*
> > + * Right now, no XSS states are used on x86 platform,
> > + * expand the macro for new features.
> > + */
> > +#define KVM_SUPPORTED_XSS      (0)
> > +
> 
> Nit: superfluous parentheses.
> 
> >  extern u64 host_xcr0;
> >
> >  extern u64 kvm_supported_xcr0(void);
> > --
> > 2.17.2
> >