From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932228AbbKLQSN (ORCPT ); Thu, 12 Nov 2015 11:18:13 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43785 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750802AbbKLQSM (ORCPT ); Thu, 12 Nov 2015 11:18:12 -0500 Subject: Re: [PATCH] KVM: svm: add support for RDTSCP To: Joerg Roedel References: <1447336157-3122-1-git-send-email-pbonzini@redhat.com> <20151112144509.GD13027@8bytes.org> Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org From: Paolo Bonzini X-Enigmail-Draft-Status: N1110 Message-ID: <5644BBBF.9060303@redhat.com> Date: Thu, 12 Nov 2015 17:18:07 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20151112144509.GD13027@8bytes.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/11/2015 15:45, Joerg Roedel wrote: > Hi Paolo, > > On Thu, Nov 12, 2015 at 02:49:16PM +0100, Paolo Bonzini wrote: >> RDTSCP was never supported for AMD CPUs, which nobody noticed because >> Linux does not use it. But exactly the fact that Linux does not >> use it makes the implementation very simple; we can freely trash >> MSR_TSC_AUX while running the guest. >> >> Cc: Joerg Roedel >> Signed-off-by: Paolo Bonzini >> --- >> arch/x86/kvm/svm.c | 24 +++++++++++++++++++++++- >> 1 file changed, 23 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c >> index 83a1c64..c302614 100644 >> --- a/arch/x86/kvm/svm.c >> +++ b/arch/x86/kvm/svm.c >> @@ -86,6 +86,7 @@ static const u32 host_save_user_msrs[] = { >> MSR_FS_BASE, >> #endif >> MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP, >> + MSR_TSC_AUX, >> }; >> >> #define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs) >> @@ -135,6 +136,7 @@ struct vcpu_svm { >> uint64_t asid_generation; >> uint64_t sysenter_esp; >> uint64_t sysenter_eip; >> + uint64_t tsc_aux; >> >> u64 next_rip; >> >> @@ -1238,6 +1240,9 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu) >> wrmsrl(MSR_AMD64_TSC_RATIO, tsc_ratio); >> } >> } >> + /* This assumes that the kernel never uses MSR_TSC_AUX */ >> + if (static_cpu_has(X86_FEATURE_RDTSCP)) >> + wrmsrl(MSR_TSC_AUX, svm->tsc_aux); >> } > > Hmm, you seem to still intercept MSR_TSC_AUX, is that intentional? Yes. If I didn't intercept MSR_TSC_AUX, I would have to read it into svm->tsc_aux on every svm_vcpu_put. Because writing MSR_TSC_AUX is a rare operation, I intercept the write, and update both svm->tsc_aux and the processor MSR_TSC_AUX in svm_set_msr. This is different from other host_save_msrs because the processor does not save MSR_TSC_AUX automatically in the VMCB. As remarked above, it only works because the kernel never uses RDTSCP. > Loading the guests value into the real cpu msr only makes sense to me > when the MSR is no longer intercepted. It's necessary for the correct operation of RDTSCP. Since we don't intercept RDTSCP, we need to load MSR_TSC_AUX. Paolo