From 516de6a075837ca03f3c3233eaa8933f4e8977b6 Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Tue, 16 Mar 2021 15:37:36 +0100 Subject: [PATCH] KVM: x86: hyper-v: Don't touch TSC page values when guest opted for re-enlightenment When guest opts for re-enlightenment notifications upon migration, it is in its right to assume that TSC page values never change (as they're only supposed to change upon migration and the host has to keep things as they are before it receives confirmation from the guest). This is mostly true until the guest is migrated somewhere. KVM userspace (e.g. QEMU) will trigger masterclock update by writing to HV_X64_MSR_REFERENCE_TSC, by calling KVM_SET_CLOCK,... and as TSC value and kvmclock reading drift apart (even slightly), the update causes TSC page values to change. The issue at hand is that when Hyper-V is migrated, it uses stale (cached) TSC page values to compute the difference between its own clocksource (provided by KVM) and its guests' TSC pages to program synthetic timers and in some cases, when TSC page is updated, this puts all stimer expirations in the past. This, in its turn, causes an interrupt storm and L2 guests not making much forward progress. Note, KVM doesn't fully implement re-enlightenment notification. Basically, the support for reenlightenment MSRs is just a stub and userspace is only expected to expose the feature when TSC scaling on the expected destination hosts is available. With TSC scaling, no real re-enlightenment is needed as TSC frequency doesn't change. With TSC scaling becoming ubiquitous, it likely makes little sense to fully implement re-enlightenment in KVM. Prevent TSC page from being updated after migration. In case it's not the guest who's initiating the change and when TSC page is already enabled, just keep it as it is: TSC value is supposed to be preserved across migration and TSC frequency can't change with re-enlightenment enabled. The guest is doomed anyway if any of this is not true. Signed-off-by: Vitaly Kuznetsov Message-Id: <20210316143736.964151-5-vkuznets@redhat.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/hyperv.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c index 5c0f10a2b3abe..f98370a399361 100644 --- a/arch/x86/kvm/hyperv.c +++ b/arch/x86/kvm/hyperv.c @@ -1077,6 +1077,21 @@ static bool compute_tsc_page_parameters(struct pvclock_vcpu_time_info *hv_clock, return true; } +/* + * Don't touch TSC page values if the guest has opted for TSC emulation after + * migration. KVM doesn't fully support reenlightenment notifications and TSC + * access emulation and Hyper-V is known to expect the values in TSC page to + * stay constant before TSC access emulation is disabled from guest side + * (HV_X64_MSR_TSC_EMULATION_STATUS). KVM userspace is expected to preserve TSC + * frequency and guest visible TSC value across migration (and prevent it when + * TSC scaling is unsupported). + */ +static inline bool tsc_page_update_unsafe(struct kvm_hv *hv) +{ + return (hv->hv_tsc_page_status != HV_TSC_PAGE_GUEST_CHANGED) && + hv->hv_tsc_emulation_control; +} + void kvm_hv_setup_tsc_page(struct kvm *kvm, struct pvclock_vcpu_time_info *hv_clock) { @@ -1104,6 +1119,14 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm, &tsc_seq, sizeof(tsc_seq)))) goto out_err; + if (tsc_seq && tsc_page_update_unsafe(hv)) { + if (kvm_read_guest(kvm, gfn_to_gpa(gfn), &hv->tsc_ref, sizeof(hv->tsc_ref))) + goto out_err; + + hv->hv_tsc_page_status = HV_TSC_PAGE_SET; + goto out_unlock; + } + /* * While we're computing and writing the parameters, force the * guest to use the time reference count MSR. @@ -1151,7 +1174,8 @@ void kvm_hv_invalidate_tsc_page(struct kvm *kvm) u64 gfn; if (hv->hv_tsc_page_status == HV_TSC_PAGE_BROKEN || - hv->hv_tsc_page_status == HV_TSC_PAGE_UNSET) + hv->hv_tsc_page_status == HV_TSC_PAGE_UNSET || + tsc_page_update_unsafe(hv)) return; mutex_lock(&hv->hv_lock); -- 2.39.5