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Peter Anvin" , David Woodhouse , Paul Durrant , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT Date: Tue, 31 Oct 2023 11:57:48 +0000 Message-Id: <20231031115748.622578-1-paul@xen.org> X-Mailer: git-send-email 2.39.2 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Paul Durrant Unless explicitly told to do so (by passing 'clocksource=tsc' and 'tsc=stable:socket', and then jumping through some hoops concerning potential CPU hotplug) Xen will never use TSC as its clocksource. Hence, by default, a Xen guest will not see PVCLOCK_TSC_STABLE_BIT set in either the primary or secondary pvclock memory areas. This has led to bugs in some guest kernels which only become evident if PVCLOCK_TSC_STABLE_BIT *is* set in the pvclocks. Hence, to support such guests, give the VMM a new Xen HVM config flag to tell KVM to forcibly clear the bit in the Xen pvclocks. Signed-off-by: Paul Durrant --- Documentation/virt/kvm/api.rst | 6 ++++++ arch/x86/kvm/x86.c | 28 +++++++++++++++++++++++----- arch/x86/kvm/xen.c | 3 ++- include/uapi/linux/kvm.h | 1 + 4 files changed, 32 insertions(+), 6 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 21a7578142a1..9752a01270df 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -8252,6 +8252,7 @@ PVHVM guests. Valid flags are:: #define KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL (1 << 4) #define KVM_XEN_HVM_CONFIG_EVTCHN_SEND (1 << 5) #define KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG (1 << 6) + #define KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE (1 << 7) The KVM_XEN_HVM_CONFIG_HYPERCALL_MSR flag indicates that the KVM_XEN_HVM_CONFIG ioctl is available, for the guest to set its hypercall page. @@ -8295,6 +8296,11 @@ behave more correctly, not using the XEN_RUNSTATE_UPDATE flag until/unless specifically enabled (by the guest making the hypercall, causing the VMM to enable the KVM_XEN_ATTR_TYPE_RUNSTATE_UPDATE_FLAG attribute). +The KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE flag indicates that KVM supports +clearing the PVCLOCK_TSC_STABLE_BIT flag in Xen pvclock sources. This will be +done when the KVM_CAP_XEN_HVM ioctl sets the +KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE flag. + 8.31 KVM_CAP_PPC_MULTITCE ------------------------- diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 41cce5031126..6abad6dacf07 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3096,7 +3096,8 @@ u64 get_kvmclock_ns(struct kvm *kvm) static void kvm_setup_guest_pvclock(struct kvm_vcpu *v, struct gfn_to_pfn_cache *gpc, - unsigned int offset) + unsigned int offset, + bool force_tsc_unstable) { struct kvm_vcpu_arch *vcpu = &v->arch; struct pvclock_vcpu_time_info *guest_hv_clock; @@ -3122,6 +3123,10 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v, */ guest_hv_clock->version = vcpu->hv_clock.version = (guest_hv_clock->version + 1) | 1; + + if (force_tsc_unstable) + guest_hv_clock->flags &= ~PVCLOCK_TSC_STABLE_BIT; + smp_wmb(); /* retain PVCLOCK_GUEST_STOPPED if set in guest copy */ @@ -3154,6 +3159,15 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) u8 pvclock_flags; bool use_master_clock; + /* + * For Xen guests we may need to override PVCLOCK_TSC_STABLE_BIT as unless + * explicitly told to use TSC as its clocksource Xen will not set this bit. + * This default behaviour led to bugs in some guest kernels which cause + * problems if they observe PVCLOCK_TSC_STABLE_BIT in the pvclock flags. + */ + bool xen_pvclock_tsc_unstable = + ka->xen_hvm_config.flags & KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE; + kernel_ns = 0; host_tsc = 0; @@ -3231,12 +3245,15 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) vcpu->hv_clock.flags = pvclock_flags; if (vcpu->pv_time.active) - kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0); + kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0, false); + if (vcpu->xen.vcpu_info_cache.active) kvm_setup_guest_pvclock(v, &vcpu->xen.vcpu_info_cache, - offsetof(struct compat_vcpu_info, time)); + offsetof(struct compat_vcpu_info, time), + xen_pvclock_tsc_unstable); if (vcpu->xen.vcpu_time_info_cache.active) - kvm_setup_guest_pvclock(v, &vcpu->xen.vcpu_time_info_cache, 0); + kvm_setup_guest_pvclock(v, &vcpu->xen.vcpu_time_info_cache, 0, + xen_pvclock_tsc_unstable); kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock); return 0; } @@ -4531,7 +4548,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL | KVM_XEN_HVM_CONFIG_SHARED_INFO | KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL | - KVM_XEN_HVM_CONFIG_EVTCHN_SEND; + KVM_XEN_HVM_CONFIG_EVTCHN_SEND | + KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE; if (sched_info_on()) r |= KVM_XEN_HVM_CONFIG_RUNSTATE | KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG; diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c index 40edf4d1974c..959061315953 100644 --- a/arch/x86/kvm/xen.c +++ b/arch/x86/kvm/xen.c @@ -1113,7 +1113,8 @@ int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc) { /* Only some feature flags need to be *enabled* by userspace */ u32 permitted_flags = KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL | - KVM_XEN_HVM_CONFIG_EVTCHN_SEND; + KVM_XEN_HVM_CONFIG_EVTCHN_SEND | + KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE; if (xhc->flags & ~permitted_flags) return -EINVAL; diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index 13065dd96132..e21b53e8358d 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -1282,6 +1282,7 @@ struct kvm_x86_mce { #define KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL (1 << 4) #define KVM_XEN_HVM_CONFIG_EVTCHN_SEND (1 << 5) #define KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG (1 << 6) +#define KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE (1 << 7) struct kvm_xen_hvm_config { __u32 flags;