Message-ID: <f22f17979d9202e2cf9580d9f82ce525645faf30.camel@infradead.org>
Date: Fri, 19 Apr 2024 13:30:42 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Dongli Zhang
	 <dongli.zhang@...cle.com>
Subject: Re: [PATCH] KVM: x86: Don't unnecessarily force masterclock update
 on vCPU hotplug

On Wed, 2023-10-18 at 12:56 -0700, 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@...hat.com>
>   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.

So what? It *should* be negative, shouldn't it? I think the problem is
how we're using that value, and what we're conflating it with.

Let us consider the case where ka->use_master_clock is true, but we're
manually upscaling the TSC in software so vcpu->tsc_catchup is also
true.

Let us postpone, for the moment, the question of whether we should even
*let* use_master_clock become true in that case.

There are a number of points in time which need to be considered:

 • vcpu->arch.this_tsc_nsec
 * kvm->arch.master_kernel_ns
 * The point in time "now" at which kvm_guest_time_update() is called.

For any given point in time, compute_guest_tsc() should calculate the
guest TSC at that moment, by scaling the elapsed microseconds since
vcpu->arch.this_tsc_nsec to the guest TSC frequency and adding that to
vcpu->arch.this_tsc_write.

I say "should", because compute_guest_tsc() is currently buggy when
asked to scale a *negative* number. Trivially fixable though.

Now, let's look at what kvm_guest_time_update() is doing. It attempts
to do two things. First it calculates the guest TSC at the reference
point that it's putting into the pvclock structure. That's what needs
to go into the 'tsc_timestamp' field of the pvclock structure alongside
the corresponding KVM clock 'system_time' at 'kernel_ns'. In master
clock mode, the value it uses for kernel_ns is ka->master_kernel_ns,
and otherwise it is the current time..

It's perfectly reasonable for master_kernel_ns to be earlier in time
than vcpu->this_tsc_nsec. That just means the TSC value we write to the
pvclock ends up being lower than the value in vcpu->this_tsc_write, by
an appropriate number of cycles. So as long as compute_guest_tsc()
isn't buggy with negative numbers, that should all be fine.

But there *is* a bug in kvm_guest_time_update(), I think...

In tsc_catchup mode, simulating a TSC which runs faster than the host,
the delta between host and guest TSCs gets larger and larger over
time. That's why kvm_guest_time_update() is called *every* time the
vCPU is entered, to adjust the TSC further and further every time.

But currently, kvm_guest_time_update() only nudges the guest TSC as far
forward as it should have been at master_kernel_ns. At any time later
than master_kernel_ns, the delta should be even higher.

I think compute_guest_tsc() should look something like this, to cope
with the negativity:

static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
{
	s64 delta = kernel_ns - vcpu->arch.this_tsc_nsec;
	u64 tsc = vcpu->arch.this_tsc_write;

	/* pvclock_scale_delta cannot cope with negative deltas */
	if (delta >= 0)
		tsc += pvclock_scale_delta(delta,
					   vcpu->arch.virtual_tsc_mult,
					   vcpu->arch.virtual_tsc_shift);
	else
		tsc -= pvclock_scale_delta(-delta,
					   vcpu->arch.virtual_tsc_mult,
					   vcpu->arch.virtual_tsc_shift);

	return tsc;
}

And the catchup code in kvm_guest_time_update() should correct *both*
the reference time *and* the current TSC by *different* amounts,
something like this:

	if (vcpu->tsc_catchup) {
		uint64_t now_guest_tsc_adjusted;
		uint64_t now_guest_tsc_unadjusted;
		int64_t now_guest_tsc_delta;

		tsc_timestamp = compute_guest_tsc(v, kernel_ns);

		if (use_master_clock) {
			uint64_t now_host_tsc;
			int64_t now_kernel_ns;

			if (!kvm_get_time_and_clockread(&now_kernel_ns, &now_host_tsc)) {
				now_kernel_ns = get_kvmclock_base_ns();
				now_host_tsc = rdtsc();
			}
			now_guest_tsc_adjusted = compute_guest_tsc(v, now_kernel_ns);
			now_guest_tsc_unadjusted = kvm_read_l1_tsc(v, now_host_tsc);
		} else {
			now_guest_tsc_adjusted = tsc_timestamp;
			now_guest_tsc_unadjusted = kvm_read_l1_tsc(v, kernel_ns);
		}

		now_guest_tsc_delta = now_guest_tsc_adjusted -
			now_guest_tsc_unadjusted;

		if (now_guest_tsc_delta > 0)
			adjust_tsc_offset_guest(v, now_guest_tsc_delta);
	} else {
		tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
	}

Then we can drop that extra masterclock update in
kvm_track_tsc_matching(), along with the comment that
compute_guest_tsc() needs the masterclock snapshot to be newer.

> 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@...cle.com>
> Closes: https://lore.kernel.org/all/20230926230649.67852-1-dongli.zhang@oracle.com
> Fixes: 7f187922ddf6 ("KVM: x86: update masterclock values on TSC writes")
> Cc: David Woodhouse <dwmw2@...radead.org>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  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


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