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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id z17sm8270044eju.27.2021.03.29.05.46.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Mar 2021 05:46:32 -0700 (PDT) From: Vitaly Kuznetsov To: "Dr. David Alan Gilbert" Subject: Re: [PATCH v2] i386: Make migration fail when Hyper-V reenlightenment was enabled but 'user_tsc_khz' is unset In-Reply-To: References: <20210319123801.1111090-1-vkuznets@redhat.com> Date: Mon, 29 Mar 2021 14:46:31 +0200 Message-ID: <87y2e69lg8.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vkuznets@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=vkuznets@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , Marcelo Tosatti , Richard Henderson , qemu-devel@nongnu.org, Eduardo Habkost Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" "Dr. David Alan Gilbert" writes: > * Vitaly Kuznetsov (vkuznets@redhat.com) wrote: >> KVM doesn't fully support Hyper-V reenlightenment notifications on >> migration. In particular, it doesn't support emulating TSC frequency >> of the source host by trapping all TSC accesses so unless TSC scaling >> is supported on the destination host and KVM_SET_TSC_KHZ succeeds, it >> is unsafe to proceed with migration. >> >> KVM_SET_TSC_KHZ is called from two sites: kvm_arch_init_vcpu() and >> kvm_arch_put_registers(). The later (intentionally) doesn't propagate >> errors allowing migrations to succeed even when TSC scaling is not >> supported on the destination. This doesn't suit 're-enlightenment' >> use-case as we have to guarantee that TSC frequency stays constant. >> >> Require 'tsc-frequency=' command line option to be specified for successful >> migration when re-enlightenment was enabled by the guest. >> >> Signed-off-by: Vitaly Kuznetsov >> --- >> This patch is a successor of "[PATCH 3/3] i386: Make sure >> kvm_arch_set_tsc_khz() succeeds on migration when 'hv-reenlightenment' >> was exposed" taking a different approach suggested by Paolo. >> --- >> docs/hyperv.txt | 5 +++++ >> target/i386/kvm/hyperv-proto.h | 1 + >> target/i386/machine.c | 20 ++++++++++++++++++++ >> 3 files changed, 26 insertions(+) >> >> diff --git a/docs/hyperv.txt b/docs/hyperv.txt >> index 5df00da54fc4..e53c581f4586 100644 >> --- a/docs/hyperv.txt >> +++ b/docs/hyperv.txt >> @@ -160,6 +160,11 @@ the hypervisor) until it is ready to switch to the new one. This, in conjunction >> with hv-frequencies, allows Hyper-V on KVM to pass stable clocksource (Reference >> TSC page) to its own guests. >> >> +Note, KVM doesn't fully support re-enlightenment notifications and doesn't >> +emulate TSC accesses after migration so 'tsc-frequency=' CPU option also has to >> +be specified to make migration succeed. The destination host has to either have >> +the same TSC frequency or support TSC scaling CPU feature. >> + >> Recommended: hv-frequencies >> >> 3.16. hv-evmcs >> diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h >> index 056a305be38c..e30d64b4ade4 100644 >> --- a/target/i386/kvm/hyperv-proto.h >> +++ b/target/i386/kvm/hyperv-proto.h >> @@ -139,6 +139,7 @@ >> * Reenlightenment notification MSRs >> */ >> #define HV_X64_MSR_REENLIGHTENMENT_CONTROL 0x40000106 >> +#define HV_REENLIGHTENMENT_ENABLE_BIT (1u << 16) >> #define HV_X64_MSR_TSC_EMULATION_CONTROL 0x40000107 >> #define HV_X64_MSR_TSC_EMULATION_STATUS 0x40000108 >> >> diff --git a/target/i386/machine.c b/target/i386/machine.c >> index 7259fe6868c6..137604ddb898 100644 >> --- a/target/i386/machine.c >> +++ b/target/i386/machine.c >> @@ -883,11 +883,31 @@ static bool hyperv_reenlightenment_enable_needed(void *opaque) >> env->msr_hv_tsc_emulation_status != 0; >> } >> >> +static int hyperv_reenlightenment_post_load(void *opaque, int version_id) >> +{ >> + X86CPU *cpu = opaque; >> + CPUX86State *env = &cpu->env; >> + >> + /* >> + * KVM doesn't fully support re-enlightenment notifications so we need to >> + * make sure TSC frequency doesn't change upon migration. >> + */ >> + if ((env->msr_hv_reenlightenment_control & HV_REENLIGHTENMENT_ENABLE_BIT) && >> + !env->user_tsc_khz) { >> + error_report("Guest enabled re-enlightenment notifications, " >> + "'tsc-frequency=' has to be specified"); > > It's unusual to fail on the destination for a valid configuration but > guest state; wouldn't it be better to always insist on tsc-frequency if > that hv feature is exposed; failing early before reeiving the state? > Doing so would make a number of currently existing configurations invalid, even when re-enlightenment is not to be used by the guest. AFAIR Windows without Hyper-V doesn't enable it. Generally, we just advise people to 'enable all currently supported hyper-v enlightenments' to make things easier so reenlightenment may end up being added for no particular reason. FWIW, v1 of this patch used a slightly different approach: I was requiring kvm_arch_set_tsc_khz() to succeed. This is not exactly the same as e.g. even when the destination host doesn't support TSC scaling, it may happen to have the same TSC frequency. -- Vitaly