From 15038e14724799b8c205beb5f20f9e54896013c3 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini <pbonzini@redhat.com> Date: Thu, 26 Oct 2017 09:13:27 +0200 Subject: KVM: SVM: obey guest PAT MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit For many years some users of assigned devices have reported worse performance on AMD processors with NPT than on AMD without NPT, Intel or bare metal. The reason turned out to be that SVM is discarding the guest PAT setting and uses the default (PA0=PA4=WB, PA1=PA5=WT, PA2=PA6=UC-, PA3=UC). The guest might be using a different setting, and especially might want write combining but isn't getting it (instead getting slow UC or UC- accesses). Thanks a lot to geoff@hostfission.com for noticing the relation to the g_pat setting. The patch has been tested also by a bunch of people on VFIO users forums. Fixes: 709ddebf81cb40e3c36c6109a7892e8b93a09464 Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=196409 Cc: stable@vger.kernel.org Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: David Hildenbrand <david@redhat.com> Tested-by: Nick Sarnie <commendsarnex@gmail.com> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com> --- arch/x86/kvm/svm.c | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'arch') diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c index b71daed3cca2..59e13a79c2e3 100644 --- a/arch/x86/kvm/svm.c +++ b/arch/x86/kvm/svm.c @@ -3671,6 +3671,13 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) u32 ecx = msr->index; u64 data = msr->data; switch (ecx) { + case MSR_IA32_CR_PAT: + if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data)) + return 1; + vcpu->arch.pat = data; + svm->vmcb->save.g_pat = data; + mark_dirty(svm->vmcb, VMCB_NPT); + break; case MSR_IA32_TSC: kvm_write_tsc(vcpu, msr); break; -- cgit v1.2.3