[PATCH v5] KVM x86/xen: add an override for PVCLOCK_TSC_STABLE_BIT

From: Paul Durrant
Date: Thu Nov 02 2023 - 12:22:05 EST

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From: Paul Durrant <pdurrant@xxxxxxxxxx>

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@xxxxxxxxxx>
---

v5:
- Fix warning reported by kernel test robot.

v4:
- Re-base.
- Re-work 'update_pvclock' test as requested.

v3:
- Moved clearing of PVCLOCK_TSC_STABLE_BIT the right side of the
memcpy().
- Added an all-vCPUs KVM_REQ_CLOCK_UPDATE when the HVM config
flag is changed.
---
Documentation/virt/kvm/api.rst | 6 ++++++
arch/x86/kvm/x86.c | 28 +++++++++++++++++++++++-----
arch/x86/kvm/xen.c | 9 ++++++++-
include/uapi/linux/kvm.h | 1 +
4 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 7025b3751027..a9bdd25826d1 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8374,6 +8374,7 @@ 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.
@@ -8417,6 +8418,11 @@ 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 2c924075f6f1..cc8d1ae29be3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -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;
@@ -4638,7 +4655,8 @@ 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 e53fad915a62..e43948b87f94 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -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;
}

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 211b86de35ac..ae90294456df 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1291,6 +1291,7 @@ 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;

base-commit: 45b890f7689eb0aba454fc5831d2d79763781677
--
2.39.2

Next message: Mario Limonciello: "Re: [PATCH 2/2] PCI: Ignore PCIe ports used for tunneling in pcie_bandwidth_available()"
Previous message: Philip Li: "Re: kernel/bpf/helpers.c:1905:19: warning: no previous declaration for 'bpf_percpu_obj_new_impl'"
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