RE: [PATCH v5] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info) sub-leaves, if present

[Durrant, Paul]

> -----Original Message-----
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: 12 July 2022 00:44
> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; David Woodhouse
> <dwmw2@xxxxxxxxxxxxx>; Paolo Bonzini <pbonzini@xxxxxxxxxx>; Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>;
> Wanpeng Li <wanpengli@xxxxxxxxxxx>; Jim Mattson <jmattson@xxxxxxxxxx>; Joerg Roedel <joro@xxxxxxxxxx>;
> Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>;
> Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>
> Subject: RE: [EXTERNAL][PATCH v5] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info) sub-leaves, if
> present
> 
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> 
> 
> On Wed, Jun 29, 2022, Paul Durrant wrote:
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 88a3026ee163..abb0a39f60eb 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -638,6 +638,7 @@ struct kvm_vcpu_xen {
> >       struct hrtimer timer;
> >       int poll_evtchn;
> >       struct timer_list poll_timer;
> > +     u32 cpuid_tsc_info;
> 
> I would prefer to follow vcpu->arch.kvm_cpuid_base and capture the base CPUID
> function.  I have a hard time believing this will be the only case where KVM needs
> to query XEN CPUID leafs.  And cpuid_tsc_info is a confusing name given the helper
> kvm_xen_setup_tsc_info(); it's odd to see a "setup" helper immediately consume a
> variable with the same name.

Sure. It is rather shrink-to-fit at the moment... no problem with capturing the base.

> 
> It'll incur another CPUID lookup in the update path to check the limit, but again
> that should be a rare operation so it doesn't seem too onerous.
> 

We could capture the limit leaf in the general case. It's not Xen-specific after all.

> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 031678eff28e..29ed665c51db 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3110,6 +3110,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> >                                  &vcpu->hv_clock.tsc_shift,
> >                                  &vcpu->hv_clock.tsc_to_system_mul);
> >               vcpu->hw_tsc_khz = tgt_tsc_khz;
> > +             kvm_xen_setup_tsc_info(v);
> 
> Any objection to s/setup/update?  KVM Xen uses "setup" for things like configuring
> the event channel using userspace input, whereas this is purely updating existing
> data structures.
> 

Sure.

> >       }
> >
> >       vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index 610beba35907..c84424d5c8b6 100644
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -10,6 +10,9 @@
> >  #include "xen.h"
> >  #include "hyperv.h"
> >  #include "lapic.h"
> > +#include "cpuid.h"
> > +
> > +#include <asm/xen/cpuid.h>
> >
> >  #include <linux/eventfd.h>
> >  #include <linux/kvm_host.h>
> > @@ -1855,3 +1858,51 @@ void kvm_xen_destroy_vm(struct kvm *kvm)
> >       if (kvm->arch.xen_hvm_config.msr)
> >               static_branch_slow_dec_deferred(&kvm_xen_enabled);
> >  }
> > +
> > +void kvm_xen_after_set_cpuid(struct kvm_vcpu *vcpu)
> > +{
> > +     u32 base = 0;
> > +     u32 limit;
> > +     u32 function;
> > +
> > +     vcpu->arch.xen.cpuid_tsc_info = 0;
> > +
> > +     for_each_possible_hypervisor_cpuid_base(function) {
> > +             struct kvm_cpuid_entry2 *entry = kvm_find_cpuid_entry(vcpu, function, 0);
> > +
> > +             if (entry &&
> > +                 entry->ebx == XEN_CPUID_SIGNATURE_EBX &&
> > +                 entry->ecx == XEN_CPUID_SIGNATURE_ECX &&
> > +                 entry->edx == XEN_CPUID_SIGNATURE_EDX) {
> > +                     base = function;
> > +                     limit = entry->eax;
> > +                     break;
> > +             }
> > +     }
> > +     if (!base)
> > +             return;
> 
> Rather than open code a variant of kvm_update_kvm_cpuid_base(), that helper can
> be tweaked to take a signature.  Along with a patch to provide a #define for Xen's
> signature as a string, this entire function becomes a one-liner.
> 

Sure, but as said above, we could make capturing the limit part of the general function too. It could even be extended to capture the Hyper-V base/limit too.
As for defining the sig as a string... I guess it would be neater to use the values from the Xen header, but it'll probably make the code more ugly so a secondary definition is reasonable.

> If the below looks ok (won't compile, needs prep patches), I'll test and post a
> proper mini-series.

Ok. Thanks,

  Paul

> 
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/cpuid.c            |  2 ++
>  arch/x86/kvm/x86.c              |  1 +
>  arch/x86/kvm/xen.c              | 30 ++++++++++++++++++++++++++++++
>  arch/x86/kvm/xen.h              | 22 +++++++++++++++++++++-
>  5 files changed, 55 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index de5a149d0971..b2565d05fc86 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -638,6 +638,7 @@ struct kvm_vcpu_xen {
>         struct hrtimer timer;
>         int poll_evtchn;
>         struct timer_list poll_timer;
> +       u32 cpuid_base;
>  };
> 
>  struct kvm_vcpu_arch {
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0abe3adc9ae3..54ed51799b8d 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -25,6 +25,7 @@
>  #include "mmu.h"
>  #include "trace.h"
>  #include "pmu.h"
> +#include "xen.h"
> 
>  /*
>   * Unlike "struct cpuinfo_x86.x86_capability", kvm_cpu_caps doesn't need to be
> @@ -309,6 +310,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>             __cr4_reserved_bits(guest_cpuid_has, vcpu);
> 
>         kvm_hv_set_cpuid(vcpu);
> +       kvm_xen_after_set_cpuid(vcpu);
> 
>         /* Invoke the vendor callback only after the above state is updated. */
>         static_call(kvm_x86_vcpu_after_set_cpuid)(vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 567d13405445..a624293c66c8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3110,6 +3110,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>                                    &vcpu->hv_clock.tsc_shift,
>                                    &vcpu->hv_clock.tsc_to_system_mul);
>                 vcpu->hw_tsc_khz = tgt_tsc_khz;
> +               kvm_xen_update_tsc_info(v);
>         }
> 
>         vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 610beba35907..3fc0c194b813 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -10,6 +10,9 @@
>  #include "xen.h"
>  #include "hyperv.h"
>  #include "lapic.h"
> +#include "cpuid.h"
> +
> +#include <asm/xen/cpuid.h>
> 
>  #include <linux/eventfd.h>
>  #include <linux/kvm_host.h>
> @@ -1855,3 +1858,30 @@ void kvm_xen_destroy_vm(struct kvm *kvm)
>         if (kvm->arch.xen_hvm_config.msr)
>                 static_branch_slow_dec_deferred(&kvm_xen_enabled);
>  }
> +
> +void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)
> +{
> +       struct kvm_cpuid_entry2 *entry;
> +       u32 function;
> +
> +       if (!vcpu->arch.xen.cpuid_base)
> +               return;
> +
> +       entry = kvm_find_cpuid_entry(vcpu, vcpu->arch.xen.cpuid_base, 0);
> +       if (WARN_ON_ONCE(!entry))
> +               return;
> +
> +       function = vcpu->arch.xen.cpuid_base | XEN_CPUID_LEAF(3);
> +       if (function > entry->eax)
> +               return;
> +
> +       entry = kvm_find_cpuid_entry(vcpu, function, 1);
> +       if (entry) {
> +               entry->ecx = vcpu->arch.hv_clock.tsc_to_system_mul;
> +               entry->edx = vcpu->arch.hv_clock.tsc_shift;
> +       }
> +
> +       entry = kvm_find_cpuid_entry(vcpu, function, 2);
> +       if (entry)
> +               entry->eax = vcpu->arch.hw_tsc_khz;
> +}
> diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
> index 532a535a9e99..b8161b99b82a 100644
> --- a/arch/x86/kvm/xen.h
> +++ b/arch/x86/kvm/xen.h
> @@ -9,9 +9,14 @@
>  #ifndef __ARCH_X86_KVM_XEN_H__
>  #define __ARCH_X86_KVM_XEN_H__
> 
> -#ifdef CONFIG_KVM_XEN
>  #include <linux/jump_label_ratelimit.h>
> 
> +#include <asm/xen/cpuid.h>
> +
> +#include "cpuid.h"
> +
> +#ifdef CONFIG_KVM_XEN
> +
>  extern struct static_key_false_deferred kvm_xen_enabled;
> 
>  int __kvm_xen_has_interrupt(struct kvm_vcpu *vcpu);
> @@ -32,6 +37,13 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe,
>  int kvm_xen_setup_evtchn(struct kvm *kvm,
>                          struct kvm_kernel_irq_routing_entry *e,
>                          const struct kvm_irq_routing_entry *ue);
> +void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu);
> +
> +static inline void kvm_xen_after_set_cpuid(struct kvm_vcpu *vcpu)
> +{
> +       vcpu->arch.xen.cpuid_base =
> +               kvm_get_hypervisor_cpuid_base(vcpu, XEN_CPUID_SIGNATURE);
> +}
> 
>  static inline bool kvm_xen_msr_enabled(struct kvm *kvm)
>  {
> @@ -135,6 +147,14 @@ static inline bool kvm_xen_timer_enabled(struct kvm_vcpu *vcpu)
>  {
>         return false;
>  }
> +
> +static inline void kvm_xen_after_set_cpuid(struct kvm_vcpu *vcpu)
> +{
> +}
> +
> +static inline void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)
> +{
> +}
>  #endif
> 
>  int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
> 
> base-commit: b08b2f54c49d8f96a22107c444d500dff73ec2a6
> --