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KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT

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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 <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20231102162128.2353459-1-paul@xen.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Paul Durrant 2023-11-02 16:21:28 +00:00 committed by Sean Christopherson
parent e9e60c82fe
commit 6d72283526
4 changed files with 38 additions and 6 deletions
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6
Documentation/virt/kvm/api.rst
View File

@ -8562,6 +8562,7 @@ 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.
@ -8605,6 +8606,11 @@ 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
-------------------------

28
arch/x86/kvm/x86.c
View File

@ -3104,7 +3104,8 @@ 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;
@ -3141,6 +3142,10 @@ 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;
@ -3161,6 +3166,16 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
u64 tsc_timestamp, host_tsc;
u8 pvclock_flags;
bool use_master_clock;
#ifdef CONFIG_KVM_XEN
/*
* 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;
#endif
kernel_ns = 0;
host_tsc = 0;
@ -3239,13 +3254,15 @@ 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);
#ifdef CONFIG_KVM_XEN
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);
#endif
kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
return 0;
@ -4646,7 +4663,8 @@ 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;

9
arch/x86/kvm/xen.c
View File

@ -1162,7 +1162,9 @@ 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;
u32 old_flags;
if (xhc->flags & ~permitted_flags)
return -EINVAL;
@ -1183,9 +1185,14 @@ int kvm_xen_hvm_config(struct kvm *kvm, struct kvm_xen_hvm_config *xhc)
else if (!xhc->msr && kvm->arch.xen_hvm_config.msr)
static_branch_slow_dec_deferred(&kvm_xen_enabled);
old_flags = kvm->arch.xen_hvm_config.flags;
memcpy(&kvm->arch.xen_hvm_config, xhc, sizeof(*xhc));
mutex_unlock(&kvm->arch.xen.xen_lock);
if ((old_flags ^ xhc->flags) & KVM_XEN_HVM_CONFIG_PVCLOCK_TSC_UNSTABLE)
kvm_make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE);
return 0;
}

1
include/uapi/linux/kvm.h
View File

@ -1318,6 +1318,7 @@ 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;