57 lines
2.0 KiB
Diff
57 lines
2.0 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Thu, 26 Oct 2017 09:13:27 +0200
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Subject: [PATCH] KVM: SVM: obey guest PAT
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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For many years some users of assigned devices have reported worse
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performance on AMD processors with NPT than on AMD without NPT,
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Intel or bare metal.
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The reason turned out to be that SVM is discarding the guest PAT
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setting and uses the default (PA0=PA4=WB, PA1=PA5=WT, PA2=PA6=UC-,
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PA3=UC). The guest might be using a different setting, and
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especially might want write combining but isn't getting it
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(instead getting slow UC or UC- accesses).
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Thanks a lot to geoff@hostfission.com for noticing the relation
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to the g_pat setting. The patch has been tested also by a bunch
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of people on VFIO users forums.
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Fixes: 709ddebf81cb40e3c36c6109a7892e8b93a09464
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Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=196409
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Cc: stable@vger.kernel.org
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Reviewed-by: David Hildenbrand <david@redhat.com>
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Tested-by: Nick Sarnie <commendsarnex@gmail.com>
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Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
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(cherry picked from commit 15038e14724799b8c205beb5f20f9e54896013c3)
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Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
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---
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arch/x86/kvm/svm.c | 7 +++++++
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1 file changed, 7 insertions(+)
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diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
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index af256b786a70..af09baa3d736 100644
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--- a/arch/x86/kvm/svm.c
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+++ b/arch/x86/kvm/svm.c
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@@ -3626,6 +3626,13 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
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u32 ecx = msr->index;
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u64 data = msr->data;
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switch (ecx) {
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+ case MSR_IA32_CR_PAT:
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+ if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
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+ return 1;
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+ vcpu->arch.pat = data;
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+ svm->vmcb->save.g_pat = data;
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+ mark_dirty(svm->vmcb, VMCB_NPT);
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+ break;
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case MSR_IA32_TSC:
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kvm_write_tsc(vcpu, msr);
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break;
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--
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2.14.2
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