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

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Message-ID: <a5d69c3b-5b9f-4ecf-bae2-2110e52eac64@xen.org>
Date: Wed, 22 Jan 2025 17:19:35 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>,
 Fred Griffoul <fgriffo@...zon.co.uk>, kvm@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, David Woodhouse <dwmw2@...radead.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Update Xen-specific CPUID leaves during
 mangling

On 22/01/2025 17:16, Vitaly Kuznetsov wrote:
> Fred Griffoul <fgriffo@...zon.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@...zon.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@...hat.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@...zon.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);
>