Re: [PATCH] KVM: x86: Don't unnecessarily force masterclock update on vCPU hotplug

[Dongli Zhang]

Tested-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>


I did the test with below KVM patch, to calculate the kvmclock at the hypervisor
side.

---
 arch/x86/kvm/x86.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b0c47b4..9ddc437 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3068,6 +3068,11 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	u64 tsc_timestamp, host_tsc;
 	u8 pvclock_flags;
 	bool use_master_clock;
+	struct pvclock_vcpu_time_info old_hv_clock;
+	u64 tsc, old_ns, new_ns, diff;
+	bool backward;
+
+	memcpy(&old_hv_clock, &vcpu->hv_clock, sizeof(old_hv_clock));

 	kernel_ns = 0;
 	host_tsc = 0;
@@ -3144,6 +3149,25 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)

 	vcpu->hv_clock.flags = pvclock_flags;

+	tsc = rdtsc();
+	tsc = kvm_read_l1_tsc(v, tsc);
+	old_ns = __pvclock_read_cycles(&old_hv_clock, tsc);
+	new_ns = __pvclock_read_cycles(&vcpu->hv_clock, tsc);
+	if (old_ns > new_ns) {
+		backward = true;
+		diff = old_ns - new_ns;
+	} else {
+		backward = false;
+		diff = new_ns - old_ns;
+	}
+	pr_alert("orabug: kvm_guest_time_update() vcpu=%d, tsc=%llu, backward=%d,
diff=%llu, old_ns=%llu, new_ns=%llu\n"
+		 "old (%u, %llu, %llu, %u, %d, %u), new (%u, %llu, %llu, %u, %d, %u)",
+		 v->vcpu_id, tsc, backward, diff, old_ns, new_ns,
+		 old_hv_clock.version, old_hv_clock.tsc_timestamp, old_hv_clock.system_time,
+		 old_hv_clock.tsc_to_system_mul, old_hv_clock.tsc_shift, old_hv_clock.flags,
+		 vcpu->hv_clock.version, vcpu->hv_clock.tsc_timestamp,
vcpu->hv_clock.system_time,
+		 vcpu->hv_clock.tsc_to_system_mul, vcpu->hv_clock.tsc_shift,
vcpu->hv_clock.flags);
+
 	if (vcpu->pv_time.active)
 		kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0);
 	if (vcpu->xen.vcpu_info_cache.active)
--

Dongli Zhang

On 10/18/23 12:56, Sean Christopherson wrote:
> Don't force a masterclock update when a vCPU synchronizes to the current
> TSC generation, e.g. when userspace hotplugs a pre-created vCPU into the
> VM.  Unnecessarily updating the masterclock is undesirable as it can cause
> kvmclock's time to jump, which is particularly painful on systems with a
> stable TSC as kvmclock _should_ be fully reliable on such systems.
> 
> The unexpected time jumps are due to differences in the TSC=>nanoseconds
> conversion algorithms between kvmclock and the host's CLOCK_MONOTONIC_RAW
> (the pvclock algorithm is inherently lossy).  When updating the
> masterclock, KVM refreshes the "base", i.e. moves the elapsed time since
> the last update from the kvmclock/pvclock algorithm to the
> CLOCK_MONOTONIC_RAW algorithm.  Synchronizing kvmclock with
> CLOCK_MONOTONIC_RAW is the lesser of evils when the TSC is unstable, but
> adds no real value when the TSC is stable.
> 
> Prior to commit 7f187922ddf6 ("KVM: x86: update masterclock values on TSC
> writes"), KVM did NOT force an update when synchronizing a vCPU to the
> current generation.
> 
>   commit 7f187922ddf6b67f2999a76dcb71663097b75497
>   Author: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
>   Date:   Tue Nov 4 21:30:44 2014 -0200
> 
>     KVM: x86: update masterclock values on TSC writes
> 
>     When the guest writes to the TSC, the masterclock TSC copy must be
>     updated as well along with the TSC_OFFSET update, otherwise a negative
>     tsc_timestamp is calculated at kvm_guest_time_update.
> 
>     Once "if (!vcpus_matched && ka->use_master_clock)" is simplified to
>     "if (ka->use_master_clock)", the corresponding "if (!ka->use_master_clock)"
>     becomes redundant, so remove the do_request boolean and collapse
>     everything into a single condition.
> 
> Before that, KVM only re-synced the masterclock if the masterclock was
> enabled or disabled  Note, at the time of the above commit, VMX
> synchronized TSC on *guest* writes to MSR_IA32_TSC:
> 
>         case MSR_IA32_TSC:
>                 kvm_write_tsc(vcpu, msr_info);
>                 break;
> 
> which is why the changelog specifically says "guest writes", but the bug
> that was being fixed wasn't unique to guest write, i.e. a TSC write from
> the host would suffer the same problem.
> 
> So even though KVM stopped synchronizing on guest writes as of commit
> 0c899c25d754 ("KVM: x86: do not attempt TSC synchronization on guest
> writes"), simply reverting commit 7f187922ddf6 is not an option.  Figuring
> out how a negative tsc_timestamp could be computed requires a bit more
> sleuthing.
> 
> In kvm_write_tsc() (at the time), except for KVM's "less than 1 second"
> hack, KVM snapshotted the vCPU's current TSC *and* the current time in
> nanoseconds, where kvm->arch.cur_tsc_nsec is the current host kernel time
> in nanoseconds:
> 
>         ns = get_kernel_ns();
> 
>         ...
> 
>         if (usdiff < USEC_PER_SEC &&
>             vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
>                 ...
>         } else {
>                 /*
>                  * We split periods of matched TSC writes into generations.
>                  * For each generation, we track the original measured
>                  * nanosecond time, offset, and write, so if TSCs are in
>                  * sync, we can match exact offset, and if not, we can match
>                  * exact software computation in compute_guest_tsc()
>                  *
>                  * These values are tracked in kvm->arch.cur_xxx variables.
>                  */
>                 kvm->arch.cur_tsc_generation++;
>                 kvm->arch.cur_tsc_nsec = ns;
>                 kvm->arch.cur_tsc_write = data;
>                 kvm->arch.cur_tsc_offset = offset;
>                 matched = false;
>                 pr_debug("kvm: new tsc generation %llu, clock %llu\n",
>                          kvm->arch.cur_tsc_generation, data);
>         }
> 
>         ...
> 
>         /* Keep track of which generation this VCPU has synchronized to */
>         vcpu->arch.this_tsc_generation = kvm->arch.cur_tsc_generation;
>         vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
>         vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
> 
> Note that the above creates a new generation and sets "matched" to false!
> But because kvm_track_tsc_matching() looks for matched+1, i.e. doesn't
> require the vCPU that creates the new generation to match itself, KVM
> would immediately compute vcpus_matched as true for VMs with a single vCPU.
> As a result, KVM would skip the masterlock update, even though a new TSC
> generation was created:
> 
>         vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
>                          atomic_read(&vcpu->kvm->online_vcpus));
> 
>         if (vcpus_matched && gtod->clock.vclock_mode == VCLOCK_TSC)
>                 if (!ka->use_master_clock)
>                         do_request = 1;
> 
>         if (!vcpus_matched && ka->use_master_clock)
>                         do_request = 1;
> 
>         if (do_request)
>                 kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
> 
> On hardware without TSC scaling support, vcpu->tsc_catchup is set to true
> if the guest TSC frequency is faster than the host TSC frequency, even if
> the TSC is otherwise stable.  And for that mode, kvm_guest_time_update(),
> by way of compute_guest_tsc(), uses vcpu->arch.this_tsc_nsec, a.k.a. the
> kernel time at the last TSC write, to compute the guest TSC relative to
> kernel time:
> 
>   static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
>   {
>         u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec,
>                                       vcpu->arch.virtual_tsc_mult,
>                                       vcpu->arch.virtual_tsc_shift);
>         tsc += vcpu->arch.this_tsc_write;
>         return tsc;
>   }
> 
> Except the "kernel_ns" passed to compute_guest_tsc() isn't the current
> kernel time, it's the masterclock snapshot!
> 
>         spin_lock(&ka->pvclock_gtod_sync_lock);
>         use_master_clock = ka->use_master_clock;
>         if (use_master_clock) {
>                 host_tsc = ka->master_cycle_now;
>                 kernel_ns = ka->master_kernel_ns;
>         }
>         spin_unlock(&ka->pvclock_gtod_sync_lock);
> 
>         if (vcpu->tsc_catchup) {
>                 u64 tsc = compute_guest_tsc(v, kernel_ns);
>                 if (tsc > tsc_timestamp) {
>                         adjust_tsc_offset_guest(v, tsc - tsc_timestamp);
>                         tsc_timestamp = tsc;
>                 }
>         }
> 
> And so when KVM skips the masterclock update after a TSC write, i.e. after
> a new TSC generation is started, the "kernel_ns-vcpu->arch.this_tsc_nsec"
> is *guaranteed* to generate a negative value, because this_tsc_nsec was
> captured after ka->master_kernel_ns.
> 
> Forcing a masterclock update essentially fudged around that problem, but
> in a heavy handed way that introduced undesirable side effects, i.e.
> unnecessarily forces a masterclock update when a new vCPU joins the party
> via hotplug.
> 
> Note, KVM forces masterclock updates in other weird ways that are also
> likely unnecessary, e.g. when establishing a new Xen shared info page and
> when userspace creates a brand new vCPU.  But the Xen thing is firmly a
> separate mess, and there are no known userspace VMMs that utilize kvmclock
> *and* create new vCPUs after the VM is up and running.  I.e. the other
> issues are future problems.
> 
> Reported-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> Closes: https://urldefense.com/v3/__https://lore.kernel.org/all/20230926230649.67852-1-dongli.zhang@oracle.com__;!!ACWV5N9M2RV99hQ!N3CdrL7gBde6tjlPxmd0cuqYCaVI4VGrvIqGX5I5pNx-cL_srMa6VuXUwrFXAA7nMgPXRvzndIOCkz-r1w$ 
> Fixes: 7f187922ddf6 ("KVM: x86: update masterclock values on TSC writes")
> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 29 ++++++++++++++++-------------
>  1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 530d4bc2259b..61bdb6c1d000 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2510,26 +2510,29 @@ static inline int gtod_is_based_on_tsc(int mode)
>  }
>  #endif
>  
> -static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
> +static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
>  {
>  #ifdef CONFIG_X86_64
> -	bool vcpus_matched;
>  	struct kvm_arch *ka = &vcpu->kvm->arch;
>  	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
>  
> -	vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
> -			 atomic_read(&vcpu->kvm->online_vcpus));
> +	/*
> +	 * To use the masterclock, the host clocksource must be based on TSC
> +	 * and all vCPUs must have matching TSCs.  Note, the count for matching
> +	 * vCPUs doesn't include the reference vCPU, hence "+1".
> +	 */
> +	bool use_master_clock = (ka->nr_vcpus_matched_tsc + 1 ==
> +				 atomic_read(&vcpu->kvm->online_vcpus)) &&
> +				gtod_is_based_on_tsc(gtod->clock.vclock_mode);
>  
>  	/*
> -	 * Once the masterclock is enabled, always perform request in
> -	 * order to update it.
> -	 *
> -	 * In order to enable masterclock, the host clocksource must be TSC
> -	 * and the vcpus need to have matched TSCs.  When that happens,
> -	 * perform request to enable masterclock.
> +	 * Request a masterclock update if the masterclock needs to be toggled
> +	 * on/off, or when starting a new generation and the masterclock is
> +	 * enabled (compute_guest_tsc() requires the masterclock snapshot to be
> +	 * taken _after_ the new generation is created).
>  	 */
> -	if (ka->use_master_clock ||
> -	    (gtod_is_based_on_tsc(gtod->clock.vclock_mode) && vcpus_matched))
> +	if ((ka->use_master_clock && new_generation) ||
> +	    (ka->use_master_clock != use_master_clock))
>  		kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
>  
>  	trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
> @@ -2706,7 +2709,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
>  	vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
>  	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
>  
> -	kvm_track_tsc_matching(vcpu);
> +	kvm_track_tsc_matching(vcpu, !matched);
>  }
>  
>  static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
> 
> base-commit: 437bba5ad2bba00c2056c896753a32edf80860cc