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

["Durrant, Paul" <pdurrant@amazon.co.uk>]

> -----Original Message-----
> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> Sent: 22 June 2022 10:40
> To: Durrant, Paul <pdurrant@amazon.co.uk>
> Cc: Paolo Bonzini <pbonzini@redhat.com>; Sean Christopherson <seanjc@google.com>; Wanpeng Li
> <wanpengli@tencent.com>; Jim Mattson <jmattson@google.com>; Joerg Roedel <joro@8bytes.org>; Thomas
> Gleixner <tglx@linutronix.de>; Ingo Molnar <mingo@redhat.com>; Borislav Petkov <bp@alien8.de>; Dave
> Hansen <dave.hansen@linux.intel.com>; H. Peter Anvin <hpa@zytor.com>; x86@kernel.org;
> kvm@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: RE: [EXTERNAL][PATCH] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info) sub-leaves, if present
> 
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> 
> 
> Paul Durrant <pdurrant@amazon.com> writes:
> 
> > The scaling information in sub-leaf 1 should match the values in the
> > 'vcpu_info' sub-structure 'time_info' (a.k.a. pvclock_vcpu_time_info) which
> > is shared with the guest. The offset values are not set since a TSC offset
> > is already applied.
> > The host TSC frequency should also be set in sub-leaf 2.
> >
> > This patch adds a new kvm_xen_set_cpuid() function that scans for the
> > relevant CPUID leaf when the CPUID information is updated by the VMM and
> > stashes pointers to the sub-leaves in the kvm_vcpu_xen structure.
> > The values are then updated by a call to the, also new,
> > kvm_xen_setup_tsc_info() function made at the end of
> > kvm_guest_time_update() just before entering the guest.
> >
> > Signed-off-by: Paul Durrant <pdurrant@amazon.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  2 ++
> >  arch/x86/kvm/cpuid.c            |  2 ++
> >  arch/x86/kvm/x86.c              |  1 +
> >  arch/x86/kvm/xen.c              | 41 +++++++++++++++++++++++++++++++++
> >  arch/x86/kvm/xen.h              | 10 ++++++++
> >  5 files changed, 56 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 1038ccb7056a..f77a4940542f 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -638,6 +638,8 @@ struct kvm_vcpu_xen {
> >       struct hrtimer timer;
> >       int poll_evtchn;
> >       struct timer_list poll_timer;
> > +     struct kvm_cpuid_entry2 *tsc_info_1;
> > +     struct kvm_cpuid_entry2 *tsc_info_2;
> >  };
> >
> >  struct kvm_vcpu_arch {
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index d47222ab8e6e..eb6cd88c974a 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
> > @@ -310,6 +311,7 @@ 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_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 00e23dc518e0..8b45f9975e45 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3123,6 +3123,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> >       if (vcpu->xen.vcpu_time_info_cache.active)
> >               kvm_setup_guest_pvclock(v, &vcpu->xen.vcpu_time_info_cache, 0);
> >       kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
> > +     kvm_xen_setup_tsc_info(v);
> >       return 0;
> >  }
> >
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index 610beba35907..a016ff85264d 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 +1858,41 @@ 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_set_cpuid(struct kvm_vcpu *vcpu)
> > +{
> > +     u32 base = 0;
> > +     u32 function;
> > +
> > +     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;
> > +                     break;
> > +             }
> > +     }
> > +     if (!base)
> > +             return;
> > +
> > +     function = base | XEN_CPUID_LEAF(3);
> > +     vcpu->arch.xen.tsc_info_1 = kvm_find_cpuid_entry(vcpu, function, 1);
> > +     vcpu->arch.xen.tsc_info_2 = kvm_find_cpuid_entry(vcpu, function, 2);
> > +}
> 
> Imagine the following scenario: CPUID data was supplied with Xen CPUID
> leaves first and then got updated with new information which doesn't
> have Xen CPUID info (e.g. has Hyper-V signature instead of Xen in the
> same 0x40000000 leaf). Won't arch.xen.tsc_info_1/arch.xen.tsc_info_2
> pointers become dangling here after we free the old CPUID data ...
> 
> > +
> > +void kvm_xen_setup_tsc_info(struct kvm_vcpu *vcpu)
> > +{
> > +     struct kvm_cpuid_entry2 *entry = vcpu->arch.xen.tsc_info_1;
> > +
> > +     if (entry) {
> > +             entry->ecx = vcpu->arch.hv_clock.tsc_to_system_mul;
> > +             entry->edx = vcpu->arch.hv_clock.tsc_shift;
> 
> ... just to crash everything here?

Yes, you are indeed correct. I'd never considered the leaves being removed from the set but the code should cope with this. V2 shortly.

  Paul

> 
> > +     }
> > +
> > +     entry = vcpu->arch.xen.tsc_info_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..1afb663318a9 100644
> > --- a/arch/x86/kvm/xen.h
> > +++ b/arch/x86/kvm/xen.h
> > @@ -32,6 +32,8 @@ 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_set_cpuid(struct kvm_vcpu *vcpu);
> > +void kvm_xen_setup_tsc_info(struct kvm_vcpu *vcpu);
> >
> >  static inline bool kvm_xen_msr_enabled(struct kvm *kvm)
> >  {
> > @@ -135,6 +137,14 @@ static inline bool kvm_xen_timer_enabled(struct kvm_vcpu *vcpu)
> >  {
> >       return false;
> >  }
> > +
> > +static inline void kvm_xen_set_cpuid(struct kvm_vcpu *vcpu)
> > +{
> > +}
> > +
> > +static inline void kvm_xen_setup_tsc_info(struct kvm_vcpu *vcpu)
> > +{
> > +}
> >  #endif
> >
> >  int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
> 
> --
> Vitaly