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Date:   Fri, 15 Oct 2021 18:58:38 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
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        Tobin Feldman-Fitzthum <tobin@ibm.com>,
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        Vlastimil Babka <vbabka@suse.cz>,
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        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH Part2 v5 43/45] KVM: SVM: Use a VMSA physical address
 variable for populating VMCB
Message-ID: <YWnPXmB4BkEO7Yrb@google.com>
References: <20210820155918.7518-1-brijesh.singh@amd.com>
 <20210820155918.7518-44-brijesh.singh@amd.com>
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On Fri, Aug 20, 2021, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> In preparation to support SEV-SNP AP Creation, use a variable that holds
> the VMSA physical address rather than converting the virtual address.
> This will allow SEV-SNP AP Creation to set the new physical address that
> will be used should the vCPU reset path be taken.

The use of "variable" in the changelog and shortlog is really confusing.  I read
them multiple times and still didn't fully understand the change until I sussed
out that the change is to track the PA in vcpu_svm separately from vcpu_svm.vmsa.

It's somewhat of a moot point though, because I think this can and should be
simplified.

In the SEV-ES case, svm->vmcb->control.vmsa_pa is always __pa(svm->vmsa).  And
in the SNP case, svm->vmcb->control.vmsa_pa defaults to __pa(svm->vmsa), but is
not changed on INIT.  Rather than do this crazy 3-way dance, simply don't write
svm->vmcb->control.vmsa_pa on INIT.  Then SNP can change it at will without having
an unnecessary and confusing field.

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 1e8b26b93b4f..0bec0b71577e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2593,13 +2593,6 @@ void sev_es_init_vmcb(struct vcpu_svm *svm)
        svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ES_ENABLE;
        svm->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK;

-       /*
-        * An SEV-ES guest requires a VMSA area that is a separate from the
-        * VMCB page. Do not include the encryption mask on the VMSA physical
-        * address since hardware will access it using the guest key.
-        */
-       svm->vmcb->control.vmsa_pa = __pa(svm->vmsa);
-
        /* Can't intercept CR register access, HV can't modify CR registers */
        svm_clr_intercept(svm, INTERCEPT_CR0_READ);
        svm_clr_intercept(svm, INTERCEPT_CR4_READ);
@@ -2633,6 +2626,13 @@ void sev_es_init_vmcb(struct vcpu_svm *svm)

 void sev_es_vcpu_reset(struct vcpu_svm *svm)
 {
+       /*
+        * An SEV-ES guest requires a VMSA area that is a separate from the
+        * VMCB page. Do not include the encryption mask on the VMSA physical
+        * address since hardware will access it using the guest key.
+        */
+       svm->vmcb->control.vmsa_pa = __pa(svm->vmsa);
+
        /*
         * Set the GHCB MSR value as per the GHCB specification when emulating
         * vCPU RESET for an SEV-ES guest.

> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>

This needs your SoB.