linux-kernel - Re: [PATCH 1/2] KVM: SVM: Fix TSC

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Message-ID: <f2c0907c-9e30-e01b-7d65-a20e6be4bf49@amd.com>
Date:   Thu, 14 Sep 2023 15:48:40 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Babu Moger <babu.moger@....com>
Subject: Re: [PATCH 1/2] KVM: SVM: Fix TSC_AUX virtualization setup

On 9/14/23 15:28, Sean Christopherson wrote:
> On Thu, Sep 14, 2023, Tom Lendacky wrote:
>> The checks for virtualizing TSC_AUX occur during the vCPU reset processing
>> path. However, at the time of initial vCPU reset processing, when the vCPU
>> is first created, not all of the guest CPUID information has been set. In
>> this case the RDTSCP and RDPID feature support for the guest is not in
>> place and so TSC_AUX virtualization is not established.
>>
>> This continues for each vCPU created for the guest. On the first boot of
>> an AP, vCPU reset processing is executed as a result of an APIC INIT
>> event, this time with all of the guest CPUID information set, resulting
>> in TSC_AUX virtualization being enabled, but only for the APs. The BSP
>> always sees a TSC_AUX value of 0 which probably went unnoticed because,
>> at least for Linux, the BSP TSC_AUX value is 0.
>>
>> Move the TSC_AUX virtualization enablement into the vcpu_after_set_cpuid()
>> path to allow for proper initialization of the support after the guest
>> CPUID information has been set.
>>
>> Fixes: 296d5a17e793 ("KVM: SEV-ES: Use V_TSC_AUX if available instead of RDTSC/MSR_TSC_AUX intercepts")
>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>> ---
>>   arch/x86/kvm/svm/sev.c | 27 +++++++++++++++++++--------
>>   arch/x86/kvm/svm/svm.c |  3 +++
>>   arch/x86/kvm/svm/svm.h |  1 +
>>   3 files changed, 23 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index b9a0a939d59f..565c9de87c6d 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -2962,6 +2962,25 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
>>   				    count, in);
>>   }
>>   
>> +static void sev_es_init_vmcb_after_set_cpuid(struct vcpu_svm *svm)
> 
> I would rather name this sev_es_after_set_cpuid() and call it directly from
> svm_vcpu_after_set_cpuid().  Or I suppose bounce through sev_after_set_cpuid(),
> but that seems gratuitous.

There is a sev_guest() check in svm_vcpu_after_set_cpuid(), so I can move 
that into sev_vcpu_after_set_cpuid() and keep the separate 
sev_es_vcpu_after_set_cpuid(). And it looks like you would prefer to not 
have "vcpu" in the function name? Might be better search-wise if vcpu 
remains part of the name?

> 
> AFAICT, there's no point in calling this from init_vmcb(); guest_cpuid_has() is
> guaranteed to be false when called during vCPU creation and so the intercept
> behavior will be correct, and even if SEV-ES called init_vmcb() from
> shutdown_interception(), which it doesn't, guest_cpuid_has() wouldn't change,
> i.e. the intercepts wouldn't need to be changed.

Ok, I thought that's how it worked, but wasn't 100% sure. I'll move it out 
of the init_vmcb() path.

> 
> init_vmcb_after_set_cpuid() is a special snowflake because it handles both SVM's
> true defaults *and* guest CPUID updates.
> 
>> +{
>> +	struct kvm_vcpu *vcpu = &svm->vcpu;
>> +
>> +	if (boot_cpu_has(X86_FEATURE_V_TSC_AUX) &&
>> +	    (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) ||
>> +	     guest_cpuid_has(vcpu, X86_FEATURE_RDPID))) {
>> +		set_msr_interception(vcpu, svm->msrpm, MSR_TSC_AUX, 1, 1);
> 
> This needs to toggled interception back on if RDTSCP and RDPID are hidden from
> the guest.  KVM's wonderful ABI doesn't disallow multiple calls to KVM_SET_CPUID2
> before KVM_RUN.

Do you want that as a separate patch with the first patch purely 
addressing the current issue? Or combine them?

> 
>> +		if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
>> +			svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> 
> Same thing here.

Will do.

Thanks,
Tom

> 
>> +	}
>> +}