Re: [PATCH 3/3] KVM: SVM: Sync GPRs to the GHCB only after VMGEXIT

[Sean Christopherson]

On Fri, Jan 22, 2021, Tom Lendacky wrote:
> On 1/22/21 5:50 PM, Sean Christopherson wrote:
> > Sync GPRs to the GHCB on VMRUN only if a sync is needed, i.e. if the
> > previous exit was a VMGEXIT and the guest is expecting some data back.
> > 
> 
> The start of sev_es_sync_to_ghcb() checks if the GHCB has been mapped, which
> only occurs on VMGEXIT, and exits early if not. And sev_es_sync_from_ghcb()
> is only called if the GHCB has been successfully mapped. The only thing in
> between is sev_es_validate_vmgexit(), which will terminate the VM on error.
> So I don't think this patch is needed.

Ah, nice!  Yep, this can be dropped.  Thanks!

Re: [PATCH 3/3] KVM: SVM: Sync GPRs to the GHCB only after VMGEXIT

[Tom Lendacky]

On 1/22/21 5:50 PM, Sean Christopherson wrote:

Sync GPRs to the GHCB on VMRUN only if a sync is needed, i.e. if the
previous exit was a VMGEXIT and the guest is expecting some data back.



The start of sev_es_sync_to_ghcb() checks if the GHCB has been mapped, 
which only occurs on VMGEXIT, and exits early if not. And 
sev_es_sync_from_ghcb() is only called if the GHCB has been successfully 
mapped. The only thing in between is sev_es_validate_vmgexit(), which will 
terminate the VM on error. So I don't think this patch is needed.


Thanks,
Tom


Cc: Brijesh Singh 
Cc: Tom Lendacky 
Signed-off-by: Sean Christopherson 
---
  arch/x86/kvm/svm/sev.c | 15 ++-
  arch/x86/kvm/svm/svm.h |  1 +
  2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index ac652bc476ae..9bd1e1650eb3 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1418,10 +1418,13 @@ static void sev_es_sync_to_ghcb(struct vcpu_svm *svm)
 * Copy their values, even if they may not have been written during the
 * VM-Exit.  It's the guest's responsibility to not consume random data.
 */
-   ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
-   ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
-   ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
-   ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
+   if (svm->need_sync_to_ghcb) {
+   ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
+   ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
+   ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
+   ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
+   svm->need_sync_to_ghcb = false;
+   }
  }
  
  static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)

@@ -1441,8 +1444,10 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
 * VMMCALL allows the guest to provide extra registers. KVM also
 * expects RSI for hypercalls, so include that, too.
 *
-* Copy their values to the appropriate location if supplied.
+* Copy their values to the appropriate location if supplied, and
+* flag that a sync back to the GHCB is needed on the next VMRUN.
 */
+   svm->need_sync_to_ghcb = true;
memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));
  
  	vcpu->arch.regs[VCPU_REGS_RAX] = ghcb_get_rax_if_valid(ghcb);

diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 0fe874ae5498..4e2e5f9fbfc2 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -192,6 +192,7 @@ struct vcpu_svm {
u64 ghcb_sa_len;
bool ghcb_sa_sync;
bool ghcb_sa_free;
+   bool need_sync_to_ghcb;
  };
  
  struct svm_cpu_data {