[PATCH 0/3] KVM: x86: Revert dirty tracking for GPRs

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>
Subject: [PATCH 0/3] KVM: x86: Revert dirty tracking for GPRs
Date: Fri, 22 Jan 2021 15:50:46 -0800	[thread overview]
Message-ID: <20210122235049.3107620-1-seanjc@google.com> (raw)

This is effectively belated feedback on the SEV-ES series.  My primary
interest is to revert the GPR dirty/available tracking, as it's pure
overhead for non-SEV-ES VMs, and even for SEV-ES I suspect the dirty
tracking is at best lost in the noise, and possibly even a net negative.

My original plan was to submit patches 1+3 as patch 1, taking a few
creative liberties with the GHCB spec to justify writing the GHCB GPRs
after every VMGEXIT.  But, since KVM is effectively writing the GHCB GPRs
on every VMRUN, I feel confident in saying that my interpretation of the
spec has already been proven correct.

The SEV-ES changes are effectively compile tested only, but unless I've
overlooked a code path, patch 1 is a nop.  Patch 3 definitely needs
testing.

Paolo, I'd really like to get patches 1 and 2 into 5.11, the code cost of
the dirty/available tracking is not trivial.

Sean Christopherson (3):
  KVM: SVM: Unconditionally sync GPRs to GHCB on VMRUN of SEV-ES guest
  KVM: x86: Revert "KVM: x86: Mark GPRs dirty when written"
  KVM: SVM: Sync GPRs to the GHCB only after VMGEXIT

 arch/x86/kvm/kvm_cache_regs.h | 51 +++++++++++++++++------------------
 arch/x86/kvm/svm/sev.c        | 14 +++++-----
 arch/x86/kvm/svm/svm.h        |  1 +
 3 files changed, 34 insertions(+), 32 deletions(-)

-- 
2.30.0.280.ga3ce27912f-goog