KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT
Commit Message
From: Paul Durrant <pdurrant@amazon.com>
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 attribute to tell KVM to forcibly
clear the bit in the Xen pvclocks.
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
---
Documentation/virt/kvm/api.rst | 9 +++++++++
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/x86.c | 25 ++++++++++++++++++++-----
arch/x86/kvm/xen.c | 13 +++++++++++++
include/uapi/linux/kvm.h | 5 +++++
5 files changed, 48 insertions(+), 5 deletions(-)
Comments
On Tue, 2023-10-10 at 09:40 +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
>
> 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 pvclock.
Specifically, some OL7 kernels backported the whole pvclock vDSO thing
but *forgot* https://git.kernel.org/torvalds/c/9f08890ab and thus kill
init with a SIGBUS the first time it tries to read a clock, because
they don't actually map the pvclock pages to userspace :)
They apparently never noticed because evidently *their* Xen fleet
doesn't actually jump through all those hoops to use the TSC as its
clocksource either.
It's a fairly safe bet that there are more broken guest kernels out
there too, hence needing to work around it.
> Hence, to support
> such guests, give the VMM a new attribute to tell KVM to forcibly
> clear the bit in the Xen pvclocks.
I frowned at the "PVCLOCK" part of the new attribute for a while,
thinking that perhaps if we're going to have a set of flags to tweak
behaviour, we shouldn't be so specific. Call it 'XEN_FEATURES' or
something... but then I realised we'd want to *advertise* the set of
bits which is available for userspace to set...
... and then I realised we already do. That's exactly what the set of
bits returned, and *set*, with KVM_CAP_XEN_HVM is for.
So let's ditch the new *attribute*, and just add your new (renamed)
KVM_XEN_HVM_CONFIG_PVCLOCK_NO_STABLE_TSC cap to the set of
permitted_flags in kvm_xen_hvm_config() so that userspace can enable it
that way like it does the INTERCEPT_HYPERCALL and EVTCHN_SEND
behaviours.
On 10/10/2023 18:32, David Woodhouse wrote:
> On Tue, 2023-10-10 at 09:40 +0000, Paul Durrant wrote:
>> From: Paul Durrant <pdurrant@amazon.com>
>>
>> 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 pvclock.
>
> Specifically, some OL7 kernels backported the whole pvclock vDSO thing
> but *forgot* https://git.kernel.org/torvalds/c/9f08890ab and thus kill
> init with a SIGBUS the first time it tries to read a clock, because
> they don't actually map the pvclock pages to userspace :)
>
> They apparently never noticed because evidently *their* Xen fleet
> doesn't actually jump through all those hoops to use the TSC as its
> clocksource either.
>
> It's a fairly safe bet that there are more broken guest kernels out
> there too, hence needing to work around it.
>
>> Hence, to support
>> such guests, give the VMM a new attribute to tell KVM to forcibly
>> clear the bit in the Xen pvclocks.
>
> I frowned at the "PVCLOCK" part of the new attribute for a while,
> thinking that perhaps if we're going to have a set of flags to tweak
> behaviour, we shouldn't be so specific. Call it 'XEN_FEATURES' or
> something... but then I realised we'd want to *advertise* the set of
> bits which is available for userspace to set...
>
> ... and then I realised we already do. That's exactly what the set of
> bits returned, and *set*, with KVM_CAP_XEN_HVM is for.
>
> So let's ditch the new *attribute*, and just add your new (renamed)
> KVM_XEN_HVM_CONFIG_PVCLOCK_NO_STABLE_TSC cap to the set of
> permitted_flags in kvm_xen_hvm_config() so that userspace can enable it
> that way like it does the INTERCEPT_HYPERCALL and EVTCHN_SEND
> behaviours.
>
Ok, sounds like a plan. I'll look at configuring it that way instead.
Paul
@@ -5544,6 +5544,7 @@ attribute cannot be read.
__u64 expires_ns;
} timer;
__u8 vector;
+ __u32 flags;
} u;
};
@@ -5610,6 +5611,14 @@ KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR
vector configured with HVM_PARAM_CALLBACK_IRQ. It is disabled by
setting the vector to zero.
+KVM_XEN_VCPU_ATTR_TYPE_PVCLOCK
+ This attribute is available when the KVM_CAP_XEN_HVM ioctl indicates
+ support for KVM_XEN_HVM_CONFIG_PVCLOCK feature. It modifies the
+ pvclock information available to the guest. Currently the only defined
+ flag is KVM_XEN_PVCLOCK_TSC_UNSTABLE. If this flag is set then the
+ PVCLOCK_TSC_STABLE_BIT flag will not be set in any of the Xen pvclock
+ sources. This aligns with Xen's behaviour when it is not using TSC
+ as its clock source, which is the default behaviour.
4.129 KVM_XEN_VCPU_GET_ATTR
---------------------------
@@ -685,6 +685,7 @@ struct kvm_vcpu_xen {
u64 hypercall_rip;
u32 current_runstate;
u8 upcall_vector;
+ bool tsc_is_unstable;
struct gfn_to_pfn_cache vcpu_info_cache;
struct gfn_to_pfn_cache vcpu_time_info_cache;
struct gfn_to_pfn_cache runstate_cache;
@@ -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;
@@ -3133,6 +3134,10 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
}
memcpy(guest_hv_clock, &vcpu->hv_clock, sizeof(*guest_hv_clock));
+
+ if (force_tsc_unstable)
+ guest_hv_clock->flags &= ~PVCLOCK_TSC_STABLE_BIT;
+
smp_wmb();
guest_hv_clock->version = ++vcpu->hv_clock.version;
@@ -3231,12 +3236,21 @@ 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);
+
+ /*
+ * 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.
+ */
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),
+ vcpu->xen.tsc_is_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,
+ vcpu->xen.tsc_is_unstable);
kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
return 0;
}
@@ -4531,7 +4545,8 @@ 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;
if (sched_info_on())
r |= KVM_XEN_HVM_CONFIG_RUNSTATE |
KVM_XEN_HVM_CONFIG_RUNSTATE_UPDATE_FLAG;
@@ -938,6 +938,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
}
break;
+ case KVM_XEN_VCPU_ATTR_TYPE_PVCLOCK:
+ vcpu->arch.xen.tsc_is_unstable = data->u.flags & KVM_XEN_PVCLOCK_TSC_UNSTABLE;
+ kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+ r = 0;
+ break;
+
default:
break;
}
@@ -1030,6 +1036,13 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
r = 0;
break;
+ case KVM_XEN_VCPU_ATTR_TYPE_PVCLOCK:
+ data->u.flags = 0;
+ if (vcpu->arch.xen.tsc_is_unstable)
+ data->u.flags |= KVM_XEN_PVCLOCK_TSC_UNSTABLE;
+ r = 0;
+ break;
+
default:
break;
}
@@ -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 (1 << 7)
struct kvm_xen_hvm_config {
__u32 flags;
@@ -1870,6 +1871,8 @@ struct kvm_xen_vcpu_attr {
__u64 expires_ns;
} timer;
__u8 vector;
+ __u32 flags;
+#define KVM_XEN_PVCLOCK_TSC_UNSTABLE (1 << 0)
} u;
};
@@ -1884,6 +1887,8 @@ struct kvm_xen_vcpu_attr {
#define KVM_XEN_VCPU_ATTR_TYPE_VCPU_ID 0x6
#define KVM_XEN_VCPU_ATTR_TYPE_TIMER 0x7
#define KVM_XEN_VCPU_ATTR_TYPE_UPCALL_VECTOR 0x8
+/* Available with KVM_CAP_XEN_HVM / KVM_XEN_HVM_CONFIG_PVCLOCK */
+#define KVM_XEN_VCPU_ATTR_TYPE_PVCLOCK 0x9
/* Secure Encrypted Virtualization command */
enum sev_cmd_id {