Re: [PATCH 1/3] KVM: SVM: Unconditionally sync GPRs to GHCB on VMRUN of SEV-ES guest

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/22/21 5:50 PM, Sean Christopherson wrote:
> Drop the per-GPR dirty checks when synchronizing GPRs to the GHCB, the
> GRPs' dirty bits are set from time zero and never cleared, i.e. will

Ah, missed that, bad assumption on my part.

> always be seen as dirty.  The obvious alternative would be to clear
> the dirty bits when appropriate, but removing the dirty checks is
> desirable as it allows reverting GPR dirty+available tracking, which
> adds overhead to all flavors of x86 VMs.
> 
> Note, unconditionally writing the GPRs in the GHCB is tacitly allowed
> by the GHCB spec, which allows the hypervisor (or guest) to provide
> unnecessary info; it's the guest's responsibility to consume only what
> it needs (the hypervisor is untrusted after all).
> 
>   The guest and hypervisor can supply additional state if desired but
>   must not rely on that additional state being provided.

Yes, that's true.

I'm ok with removing the tracking if that's desired. Otherwise, we can add
a vcpu->arch.regs_dirty = 0 in sev_es_sync_from_ghcb().

Thanks,
Tom

> 
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/svm/sev.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index c8ffdbc81709..ac652bc476ae 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1415,16 +1415,13 @@ static void sev_es_sync_to_ghcb(struct vcpu_svm *svm)
>  	 * to be returned:
>  	 *   GPRs RAX, RBX, RCX, RDX
>  	 *
> -	 * Copy their values to the GHCB if they are dirty.
> +	 * 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.
>  	 */
> -	if (kvm_register_is_dirty(vcpu, VCPU_REGS_RAX))
> -		ghcb_set_rax(ghcb, vcpu->arch.regs[VCPU_REGS_RAX]);
> -	if (kvm_register_is_dirty(vcpu, VCPU_REGS_RBX))
> -		ghcb_set_rbx(ghcb, vcpu->arch.regs[VCPU_REGS_RBX]);
> -	if (kvm_register_is_dirty(vcpu, VCPU_REGS_RCX))
> -		ghcb_set_rcx(ghcb, vcpu->arch.regs[VCPU_REGS_RCX]);
> -	if (kvm_register_is_dirty(vcpu, VCPU_REGS_RDX))
> -		ghcb_set_rdx(ghcb, vcpu->arch.regs[VCPU_REGS_RDX]);
> +	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]);
>  }
>  
>  static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
>