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

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Message-ID: <ZQSVFQ78M/OUtWaj@google.com>
Date:   Fri, 15 Sep 2023 10:32:05 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....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 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).