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

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Message-ID: <ff993ee2-fdc6-1849-4290-efd2efc6ca06@amd.com>
Date:   Fri, 15 Sep 2023 15:54:09 -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 9/15/23 12:32, Sean Christopherson wrote:
> On Fri, Sep 15, 2023, Tom Lendacky wrote:
>> On 9/14/23 15:48, Tom Lendacky wrote:
>>> On 9/14/23 15:28, Sean Christopherson wrote:
>>>> On Thu, Sep 14, 2023, Tom Lendacky wrote:
>>
>>>
>>>>
>>>>> +        if (guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
>>>>> +            svm_clr_intercept(svm, INTERCEPT_RDTSCP);
>>>>
>>>> Same thing here.
>>>
>>> Will do.
>>
>> For RDTSCP, svm_recalc_instruction_intercepts() will set/clear the RDTSCP
>> intercept as part of the svm_vcpu_set_after_cpuid() path, but it will only
>> do it based on kvm_cpu_cap_has(X86_FEATURE_RDTSCP) being true, which is very
>> likely.
>>
>> Do you think that is good enough and we can drop the setting and clearing of
>> the RDTSCP intercept in the sev_es_vcpu_set_after_cpuid() function and only
>> deal with the TSC_AUX MSR intercept?
> 
> The common handling should be good enough.
> 
>> On a side note, it looks like RDTSCP would not be intercepted if the KVM cap
>> X86_FEATURE_RDTSCP feature is cleared, however unlikely, in
>> kvm_set_cpu_caps() and RDTSCP is not advertised to the guest (assuming the
>> guest is ignoring the RDTSCP CPUID bit).
> 
> Hmm, yes, though the only scenario in which KVM clears RDTSCP on AMD comes with
> a WARN (it's a guard against KVM bugs).  If the guest ignores CPUID and uses
> RDTSCP anyways, the guest deserves its death, and leaking the host pCPU doesn't
> seem like a major issue.
> 
> That said, if hardware behavior is to ignore unknown intercepts, e.g. if KVM can
> safely set INTERCEPT_RDTSCP even when hardware doesn't support said intercept,
> then I wouldn't be opposed to doing:
> 
> 	/*
> 	 * Intercept INVPCID if shadow paging is enabled to sync/free shadow
> 	 * roots, or if INVPCID is disabled in the guest to inject #UD.
> 	 */
> 	if (!kvm_cpu_cap_has(X86_FEATURE_INVPCID) ||
> 	    !npt_enabled || !guest_cpuid_has(&svm->vcpu, X86_FEATURE_INVPCID))
> 		svm_set_intercept(svm, INTERCEPT_INVPCID);
> 	else
> 		svm_clr_intercept(svm, INTERCEPT_INVPCID);
> 
> 	if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP) &&
> 	    guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> 		svm_clr_intercept(svm, INTERCEPT_RDTSCP);
> 	else
> 		svm_set_intercept(svm, INTERCEPT_RDTSCP);
> 
> Alternatively, KVM could check boot_cpu_has() instead or kvm_cpu_cap_has(), but
> that's not foolproof either, e.g. see Intel's of hiding PCID to workaround the
> TLB flushing bug on Alderlake.  So my vote would either be to keep things as-is,
> or do the above (if that's safe).

Keep things as-is works for me :)

Thanks,
Tom