linux-kernel - Re: [PATCH kernel 5/9] KVM: SVM/SEV/SEV-ES: Rework intercepts

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Message-ID: <ZJ9N3GMNuKJXc5wQ@google.com>
Date:   Fri, 30 Jun 2023 14:49:16 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Santosh Shukla <santosh.shukla@....com>
Subject: Re: [PATCH kernel 5/9] KVM: SVM/SEV/SEV-ES: Rework intercepts

On Thu, Jun 15, 2023, Alexey Kardashevskiy wrote:
> @@ -2976,6 +2977,16 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>  	svm_set_intercept(svm, TRAP_CR4_WRITE);
>  	svm_set_intercept(svm, TRAP_CR8_WRITE);
>  
> +	/*
> +	 * DR7 access must remain intercepted for an SEV-ES guest to disallow
> +	 * the guest kernel set up a #DB on memory that's needed to vector a #DB
> +	 * as otherwise the CPU gets stuck in an infinite #DB loop.
> +	 */

This isn't correct.  Letting the guest configuring breakpoints would be weird
and nonsensical, but it wouldn't lead to infinite #DBs so long as KVM intercepts
#DB.

KVM intercepts DR7 when DebugSwap isn't enabled because otherwise KVM has no way
of context switching DR[0-3] for the guest.  At least, I assume that's the case,
AFAICT the APM never actually says what happens with DR[0-3] when DebugSwap is
disabled.