RE: [PATCH] KVM: x86: Make sure KVM_CPUID_FEATURES really are KVM_CPUID_FEATURES

[Durrant, Paul]

> -----Original Message-----
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: 04 November 2021 19:23
> To: Paul Durrant <paul@xxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Durrant, Paul <pdurrant@xxxxxxxxxxxx>; Paolo
> Bonzini <pbonzini@xxxxxxxxxx>; Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>; Wanpeng Li
> <wanpengli@xxxxxxxxxxx>; Jim Mattson <jmattson@xxxxxxxxxx>; Joerg Roedel <joro@xxxxxxxxxx>
> Subject: RE: [EXTERNAL] [PATCH] KVM: x86: Make sure KVM_CPUID_FEATURES really are KVM_CPUID_FEATURES
> 
> On Thu, Nov 04, 2021, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >
> > Currently when kvm_update_cpuid_runtime() runs, it assumes that the
> > KVM_CPUID_FEATURES leaf is located at 0x40000001. This is not true,
> > however, if Hyper-V support is enabled. In this case the KVM leaves will
> > be offset.
> >
> > This patch introdues as new 'kvm_cpuid_base' field into struct
> > kvm_vcpu_arch to track the location of the KVM leaves and function
> > kvm_update_cpuid_base() (called from kvm_update_cpuid_runtime()) to locate
> > the leaves using the 'KVMKVMKVM\0\0\0' signature. Adjustment of
> > KVM_CPUID_FEATURES will hence now target the correct leaf.
> >
> > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > ---
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> > Cc: Joerg Roedel <joro@xxxxxxxxxx>
> 
> scripts/get_maintainer.pl is your friend :-)

That's what I used, but thought it prudent to trim the list to just KVM reviewers.

> 
> > ---
> >  arch/x86/include/asm/kvm_host.h |  1 +
> >  arch/x86/kvm/cpuid.c            | 50 +++++++++++++++++++++++++++++----
> >  2 files changed, 46 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 88fce6ab4bbd..21133ffa23e9 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -725,6 +725,7 @@ struct kvm_vcpu_arch {
> >
> >       int cpuid_nent;
> >       struct kvm_cpuid_entry2 *cpuid_entries;
> > +     u32 kvm_cpuid_base;
> >
> >       u64 reserved_gpa_bits;
> >       int maxphyaddr;
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 2d70edb0f323..2cfb8ec4f570 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -99,11 +99,46 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent)
> >       return 0;
> >  }
> >
> > +static void kvm_update_cpuid_base(struct kvm_vcpu *vcpu)
> > +{
> > +     u32 function;
> > +
> > +     for (function = 0x40000000; function < 0x40010000; function += 0x100) {
> 
> No small part of me wants to turn hypervisor_cpuid_base() into a macro, but that's
> probably more pain than gain.  But I do think it would be worth providing a macro
> to iterate over possible bases and share that with the guest-side code.
> 

Ok.

> > +             struct kvm_cpuid_entry2 *best = kvm_find_cpuid_entry(vcpu, function, 0);
> 
> Declare "struct kvm_cpuid_entry2 *best" outside of the loop to shorten this line.
> I'd also vote to rename "best" to "entry".  KVM's "best" terminology is a remnant
> of misguided logic that applied Intel's bizarre out-of-range behavior to internal
> KVM lookups.
> 

Sure.

> > +
> > +             if (best) {
> > +                     char signature[12];
> > +
> > +                     *(u32 *)&signature[0] = best->ebx;
> 
> Just make signature a u32[3], then the casting craziness goes away.
> 

True.

> > +                     *(u32 *)&signature[4] = best->ecx;
> > +                     *(u32 *)&signature[8] = best->edx;
> > +
> > +                     if (!memcmp(signature, "KVMKVMKVM\0\0\0", 12))
> 
> The "KVMKVMKVM\0\0\0" magic string belongs in a #define that's shared with the
> guest-side code.  I
> 

It's in a few places, so yes a #define is a good idea.

> > +                             break;
> > +             }
> > +     }
> > +     vcpu->arch.kvm_cpuid_base = function;
> 
> Unconditionally setting kvm_cpuid_base is silly because then kvm_get_cpuid_base()
> needs to check multiple "error" values.
> 
> E.g. all of the above can be done as:
> 
>         struct kvm_cpuid_entry2 *entry;
>         u32 base, signature[3];
> 
>         vcpu->arch.kvm_cpuid_base = 0;
> 
>         virt_for_each_possible_hypervisor_base(base) {
>                 entry = kvm_find_cpuid_entry(vcpu, base, 0);
>                 if (!entry)
>                         continue;
> 
>                 signature[0] = entry->ebx;
>                 signature[1] = entry->ecx;
>                 signature[2] = entry->edx;
> 
>                 if (!memcmp(signature, KVM_CPUID_SIG, sizeof(signature))) {
>                         vcpu->arch.kvm_cpuid_base = base;
>                         break;
>                 }
>         }
> 
> > +}
> > +
> > +static inline bool kvm_get_cpuid_base(struct kvm_vcpu *vcpu, u32 *function)
> > +{
> > +     if (vcpu->arch.kvm_cpuid_base < 0x40000000 ||
> > +         vcpu->arch.kvm_cpuid_base >= 0x40010000)
> > +             return false;
> > +
> > +     *function = vcpu->arch.kvm_cpuid_base;
> > +     return true;
> 
> If '0' is the "doesn't exist" value, then this helper goes away.
> 
> > +}
> > +
> >  void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
> >  {
> > +     u32 base;
> >       struct kvm_cpuid_entry2 *best;
> >
> > -     best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
> > +     if (!kvm_get_cpuid_base(vcpu, &base))
> > +             return;
> 
> ... and then this becomes:
> 
>         if (!vcpu->arch.kvm_cpuid_base)
>                 return;
> 

Ok.

> Actually, since this is a repated pattern and is likely going to be limited to
> getting KVM_CPUID_FEATURES, just add:
> 
> struct kvm_find_cpuid_entry kvm_find_kvm_cpuid_features(void)
> {
>         u32 base = vcpu->arch.kvm_cpuid_base;
> 
>         if (!base)
>                 return NULL;
> 
>         return kvm_find_cpuid_entry(vcpu, base | KVM_CPUID_FEATURES, 0);
> }
> 
> and then all of the indentation churn goes away.
> 

Yeah, probably neater.

> > +
> > +     best = kvm_find_cpuid_entry(vcpu, base + KVM_CPUID_FEATURES, 0);
> >
> >       /*
> >        * save the feature bitmap to avoid cpuid lookup for every PV
> > @@ -116,6 +151,7 @@ void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
> >  void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> >  {
> >       struct kvm_cpuid_entry2 *best;
> > +     u32 base;
> >
> >       best = kvm_find_cpuid_entry(vcpu, 1, 0);
> >       if (best) {
> > @@ -142,10 +178,14 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> >                    cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> >               best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> >
> > -     best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
> > -     if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> > -             (best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> > -             best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> > +     kvm_update_cpuid_base(vcpu);
> 
> The KVM base doesn't need to be rechecked for runtime updates.  Runtime updates
> are to handle changes in guest state, e.g. reported XSAVE size in response to a
> CR4.OSXSAVE change.  The raw CPUID entries themselves cannot change at runtime.
> I suspect you did this here because kvm_update_cpuid_runtime() is called before
> kvm_vcpu_after_set_cpuid(), but that has the very bad side effect of doing an
> _expensive_ lookup on every runtime update, which can get very painful if there's
> no KVM_CPUID_FEATURES to be found.
> 
> If you include the prep patch (pasted at the bottom), then this can simply be
> (note the somewhat silly name; I think it's worth clarifying that it's the
> KVM_CPUID_* base that's being updated):
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0c99d2731076..5dd8c26e9f86 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -245,6 +245,7 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>         vcpu->arch.cpuid_entries = e2;
>         vcpu->arch.cpuid_nent = nent;
> 
> +       kvm_update_kvm_cpuid_base(vcpu);
>         kvm_update_cpuid_runtime(vcpu);
>         kvm_vcpu_after_set_cpuid(vcpu);
> 
> > +
> > +     if (kvm_get_cpuid_base(vcpu, &base)) {
> > +             best = kvm_find_cpuid_entry(vcpu, base + KVM_CPUID_FEATURES, 0);
> 
> This is wrong.  base will be >0x40000000 and <0x40010000, and KVM_CPUID_FEATURES
> is 0x40000001, i.e. this will lookup 0x80000001 for the default base.  The '+'
> needs to be an '|'.
> 

Yes, it does.

> > +             if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> > +                 (best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> > +                     best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> > +     }
> >
> >       if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)) {
> >               best = kvm_find_cpuid_entry(vcpu, 0x1, 0);
> > --
> > 2.20.1
> 

Thanks,

  Paul

> 
> From 02d58c124f5aab1b0ef28cfc8a6ff6b6c58df969 Mon Sep 17 00:00:00 2001
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Date: Thu, 4 Nov 2021 12:17:23 -0700
> Subject: [PATCH] KVM: x86: Add helper to consolidate core logic of
>  SET_CPUID{2} flows
> 
> Move the core logic of SET_CPUID and SET_CPUID2 to a common helper, the
> only difference between the two ioctls() is the format of the userspace
> struct.  A future fix will add yet more code to the core logic.
> 
> No functional change intended.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 47 ++++++++++++++++++++++----------------------
>  1 file changed, 24 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 751aa85a3001..0c99d2731076 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -232,6 +232,25 @@ u64 kvm_vcpu_reserved_gpa_bits_raw(struct kvm_vcpu *vcpu)
>         return rsvd_bits(cpuid_maxphyaddr(vcpu), 63);
>  }
> 
> +static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> +                        int nent)
> +{
> +       int r;
> +
> +       r = kvm_check_cpuid(e2, nent);
> +       if (r)
> +               return r;
> +
> +       kvfree(vcpu->arch.cpuid_entries);
> +       vcpu->arch.cpuid_entries = e2;
> +       vcpu->arch.cpuid_nent = nent;
> +
> +       kvm_update_cpuid_runtime(vcpu);
> +       kvm_vcpu_after_set_cpuid(vcpu);
> +
> +       return 0;
> +}
> +
>  /* when an old userspace process fills a new kernel module */
>  int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
>                              struct kvm_cpuid *cpuid,
> @@ -268,18 +287,9 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
>                 e2[i].padding[2] = 0;
>         }
> 
> -       r = kvm_check_cpuid(e2, cpuid->nent);
> -       if (r) {
> +       r = kvm_set_cpuid(vcpu, e2, cpuid->nent);
> +       if (r)
>                 kvfree(e2);
> -               goto out_free_cpuid;
> -       }
> -
> -       kvfree(vcpu->arch.cpuid_entries);
> -       vcpu->arch.cpuid_entries = e2;
> -       vcpu->arch.cpuid_nent = cpuid->nent;
> -
> -       kvm_update_cpuid_runtime(vcpu);
> -       kvm_vcpu_after_set_cpuid(vcpu);
> 
>  out_free_cpuid:
>         kvfree(e);
> @@ -303,20 +313,11 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
>                         return PTR_ERR(e2);
>         }
> 
> -       r = kvm_check_cpuid(e2, cpuid->nent);
> -       if (r) {
> +       r = kvm_set_cpuid(vcpu, e2, cpuid->nent);
> +       if (r)
>                 kvfree(e2);
> -               return r;
> -       }
> 
> -       kvfree(vcpu->arch.cpuid_entries);
> -       vcpu->arch.cpuid_entries = e2;
> -       vcpu->arch.cpuid_nent = cpuid->nent;
> -
> -       kvm_update_cpuid_runtime(vcpu);
> -       kvm_vcpu_after_set_cpuid(vcpu);
> -
> -       return 0;
> +       return r;
>  }
> 
>  int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
> --