Re: [PATCH Part2 v5 43/45] KVM: SVM: Use a VMSA physical address variable for populating VMCB

Sean Christopherson
2021-10-15T18:58:46Z

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.