Re: [PATCH] KVM: x86: Update Xen-specific CPUID leaves during mangling

Paul Durrant
2025-01-22T17:19:39Z

On 22/01/2025 17:16, Vitaly Kuznetsov wrote:
> Fred Griffoul <fgriffo@amazon.co.uk> writes:
>
>> Previous commit ee3a5f9e3d9b ("KVM: x86: Do runtime CPUID update before
>> updating vcpu->arch.cpuid_entries") implemented CPUID data mangling in
>> KVM_SET_CPUID2 support before verifying that no changes occur on running
>> vCPUs. However, it overlooked the CPUID leaves that are modified by
>> KVM's Xen emulation.
>>
>> Fix this by calling a Xen update function when mangling CPUID data.
>>
>> Fixes: ee3a5f9e3d9b ("KVM: x86: Do runtime CPUID update before
>> updating vcpu->arch.cpuid_entries")
>
> Well, kvm_xen_update_tsc_info() was added with
>
> commit f422f853af0369be27d2a9f1b20079f2bc3d1ca2
> Author: Paul Durrant <pdurrant@amazon.com>
> Date: Fri Jan 6 10:36:00 2023 +0000
>
> KVM: x86/xen: update Xen CPUID Leaf 4 (tsc info) sub-leaves, if present
>
> and the commit you mention in 'Fixes' is older:
>
> commit ee3a5f9e3d9bf94159f3cc80da542fbe83502dd8
> Author: Vitaly Kuznetsov <vkuznets@redhat.com>
> Date: Mon Jan 17 16:05:39 2022 +0100
>
> KVM: x86: Do runtime CPUID update before updating vcpu->arch.cpuid_entries
>
> so I guess we should be 'Fixing' f422f853af03 instead :-)
>
>> Signed-off-by: Fred Griffoul <fgriffo@amazon.co.uk>
>> ---
>> arch/x86/kvm/cpuid.c | 1 +
>> arch/x86/kvm/xen.c | 5 +++++
>> arch/x86/kvm/xen.h | 5 +++++
>> 3 files changed, 11 insertions(+)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index edef30359c19..432d8e9e1bab 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -212,6 +212,7 @@ static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2
>> */
>> kvm_update_cpuid_runtime(vcpu);
>> kvm_apply_cpuid_pv_features_quirk(vcpu);
>> + kvm_xen_update_cpuid_runtime(vcpu);
>
> This one is weird as we update it in runtime (kvm_guest_time_update())
> and values may change when we e.g. migrate the guest. First, I do not
> understand how the guest is supposed to notice the change as CPUID data
> is normally considered static. Second, I do not see how the VMM is
> supposed to track it as if it tries to supply some different data for
> these Xen leaves, kvm_cpuid_check_equal() will still fail.
>
> Would it make more sense to just ignore these Xen CPUID leaves with TSC
> information when we do the comparison?
>

What is the purpose of the comparison anyway? IIUC we want to ensure
that a VMM does not change its mind after KVM_RUN so should we not be
stashing what was set by the VMM and comparing against that *before*
mangling any values?

>>
>> if (nent != vcpu->arch.cpuid_nent)
>> return -EINVAL;
>> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
>> index a909b817b9c0..219f9a9a92be 100644
>> --- a/arch/x86/kvm/xen.c
>> +++ b/arch/x86/kvm/xen.c
>> @@ -2270,6 +2270,11 @@ void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)
>> entry->eax = vcpu->arch.hw_tsc_khz;
>> }
>>
>> +void kvm_xen_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>> +{
>> + kvm_xen_update_tsc_info(vcpu);
>> +}
>> +
>> void kvm_xen_init_vm(struct kvm *kvm)
>> {
>> mutex_init(&kvm->arch.xen.xen_lock);
>> diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
>> index f5841d9000ae..d3182b0ab7e3 100644
>> --- a/arch/x86/kvm/xen.h
>> +++ b/arch/x86/kvm/xen.h
>> @@ -36,6 +36,7 @@ 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);
>> +void kvm_xen_update_cpuid_runtime(struct kvm_vcpu *vcpu);
>>
>> static inline void kvm_xen_sw_enable_lapic(struct kvm_vcpu *vcpu)
>> {
>> @@ -160,6 +161,10 @@ static inline bool kvm_xen_timer_enabled(struct kvm_vcpu *vcpu)
>> static inline void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)
>> {
>> }
>> +
>> +static inline void kvm_xen_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>> +{
>> +}
>> #endif
>>
>> int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
>