Re: [PATCH v6] KVM: x86/tsc: Don't sync user-written TSC against startup values
From: Like Xu
Date: Mon Sep 25 2023 - 03:37:10 EST
Ping for further comments and confirmation from Sean.
Could we trigger a new version for this issue ? Thanks.
On 19/9/2023 7:29 pm, Like Xu wrote:
On 14/9/2023 3:31 pm, David Woodhouse wrote:
On Thu, 2023-09-14 at 11:50 +0800, Like Xu wrote:
On 13/9/2023 10:47 pm, Sean Christopherson wrote:
On Wed, Sep 13, 2023, Like Xu wrote:
I'll wait for a cooling off period to see if the maintainers need me to
post v7.
You should have waiting to post v5, let alone v6. Resurrecting a thread
after a
month and not waiting even 7 hours for others to respond is extremely
frustrating.
You are right. I don't seem to be keeping up with many of other issues. Sorry
for that.
Wish there were 48 hours in a day.
Back to this issue: for commit message, I'd be more inclined to David's
understanding,
The discussion that Sean and I had should probably be reflected in the
commit message too. To the end of the commit log you used for v6, after
the final 'To that end:…' paragraph, let's add:
Note that userspace can explicitly request a *synchronization* of the
TSC by writing zero. For the purpose of this patch, this counts as
"setting" the TSC. If userspace then subsequently writes an explicit
non-zero value which happens to be within 1 second of the previous
value, it will be 'corrected'. For that case, this preserves the prior
behaviour of KVM (which always applied the 1-second 'correction'
regardless of user vs. kernel).
@@ -2728,27 +2729,45 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu,
u64 data)
elapsed = ns - kvm->arch.last_tsc_nsec;
if (vcpu->arch.virtual_tsc_khz) {
+ /*
+ * Force synchronization when creating or hotplugging a vCPU,
+ * i.e. when the TSC value is '0', to help keep clocks stable.
+ * If this is NOT a hotplug/creation case, skip synchronization
+ * on the first write from userspace so as not to misconstrue
+ * state restoration after live migration as an attempt from
+ * userspace to synchronize.
+ */
You cannot *misconstrue* an attempt from userspace to synchronize. If
userspace writes a zero, it's a sync attempt. If it's non-zero it's a
TSC value to be set. It's not very subtle :)
I think the 1-second slop thing is sufficiently documented in the 'else
if' clause below, so I started writing an alternative 'overall' comment
to go here and found it a bit redundant. So maybe let's just drop this
comment and add one back in the if (data == 0) case...
if (data == 0) {
- /*
- * detection of vcpu initialization -- need to sync
- * with other vCPUs. This particularly helps to keep
- * kvm_clock stable after CPU hotplug
- */
/*
* Force synchronization when creating a vCPU, or when
* userspace explicitly writes a zero value.
*/
synchronizing = true;
- } else {
+ } else if (kvm->arch.user_set_tsc) {
u64 tsc_exp = kvm->arch.last_tsc_write +
nsec_to_cycles(vcpu, elapsed);
u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
/*
- * Special case: TSC write with a small delta (1 second)
- * of virtual cycle time against real time is
- * interpreted as an attempt to synchronize the CPU.
+ * Here lies UAPI baggage: when a user-initiated TSC
write has
+ * a small delta (1 second) of virtual cycle time
against the
+ * previously set vCPU, we assume that they were
intended to be
+ * in sync and the delta was only due to the racy
nature of the
+ * legacy API.
+ *
+ * This trick falls down when restoring a guest which
genuinely
+ * has been running for less time than the 1 second
of imprecision
+ * which we allow for in the legacy API. In this
case, the first
+ * value written by userspace (on any vCPU) should
not be subject
+ * to this 'correction' to make it sync up with
values that only
Missing the word 'come' here too, in '…that only *come* from…',
+ * from the kernel's default vCPU creation. Make the
1-second slop
+ * hack only trigger if the user_set_tsc flag is
already set.
+ *
+ * The correct answer is for the VMM not to use the
legacy API.
Maybe we should drop this line, as we don't actually have a sane API
yet that VMMs can use instead.
Thanks for your comments, but not sure if Sean has any more concerns to move
forward:
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 1a4def36d5bb..9a7dfef9d32d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1324,6 +1324,7 @@ struct kvm_arch {
int nr_vcpus_matched_tsc;
u32 default_tsc_khz;
+ bool user_set_tsc;
seqcount_raw_spinlock_t pvclock_sc;
bool use_master_clock;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6c9c81e82e65..11fbd2a4a370 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2714,8 +2714,9 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu,
u64 offset, u64 tsc,
kvm_track_tsc_matching(vcpu);
}
-static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
+static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
{
+ u64 data = user_value ? *user_value : 0;
struct kvm *kvm = vcpu->kvm;
u64 offset, ns, elapsed;
unsigned long flags;
@@ -2730,25 +2731,37 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu,
u64 data)
if (vcpu->arch.virtual_tsc_khz) {
if (data == 0) {
/*
- * detection of vcpu initialization -- need to sync
- * with other vCPUs. This particularly helps to keep
- * kvm_clock stable after CPU hotplug
+ * Force synchronization when creating a vCPU, or when
+ * userspace explicitly writes a zero value.
*/
synchronizing = true;
- } else {
+ } else if (kvm->arch.user_set_tsc) {
u64 tsc_exp = kvm->arch.last_tsc_write +
nsec_to_cycles(vcpu, elapsed);
u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
/*
- * Special case: TSC write with a small delta (1 second)
- * of virtual cycle time against real time is
- * interpreted as an attempt to synchronize the CPU.
+ * Here lies UAPI baggage: when a user-initiated TSC write has
+ * a small delta (1 second) of virtual cycle time against the
+ * previously set vCPU, we assume that they were intended to be
+ * in sync and the delta was only due to the racy nature of the
+ * legacy API.
+ *
+ * This trick falls down when restoring a guest which genuinely
+ * has been running for less time than the 1 second of imprecision
+ * which we allow for in the legacy API. In this case, the first
+ * value written by userspace (on any vCPU) should not be subject
+ * to this 'correction' to make it sync up with values that only
+ * come from the kernel's default vCPU creation. Make the 1-second
+ * slop hack only trigger if the user_set_tsc flag is already set.
*/
synchronizing = data < tsc_exp + tsc_hz &&
data + tsc_hz > tsc_exp;
}
}
+ if (user_value)
+ kvm->arch.user_set_tsc = true;
+
/*
* For a reliable TSC, we can match TSC offsets, and for an unstable
* TSC, we add elapsed time in this computation. We could let the
@@ -3777,7 +3790,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct
msr_data *msr_info)
break;
case MSR_IA32_TSC:
if (msr_info->host_initiated) {
- kvm_synchronize_tsc(vcpu, data);
+ kvm_synchronize_tsc(vcpu, &data);
} else {
u64 adj = kvm_compute_l1_tsc_offset(vcpu, data) -
vcpu->arch.l1_tsc_offset;
adjust_tsc_offset_guest(vcpu, adj);
@@ -5536,6 +5549,7 @@ static int kvm_arch_tsc_set_attr(struct kvm_vcpu *vcpu,
tsc = kvm_scale_tsc(rdtsc(), vcpu->arch.l1_tsc_scaling_ratio) + offset;
ns = get_kvmclock_base_ns();
+ kvm->arch.user_set_tsc = true;
__kvm_synchronize_tsc(vcpu, offset, tsc, ns, matched);
raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
@@ -11959,7 +11973,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
if (mutex_lock_killable(&vcpu->mutex))
return;
vcpu_load(vcpu);
- kvm_synchronize_tsc(vcpu, 0);
+ kvm_synchronize_tsc(vcpu, NULL);
vcpu_put(vcpu);
/* poll control enabled by default */