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

[Dongli Zhang]

Hi Sean,

Would mind sharing if the patch is waiting for Reviewed-by, and when it will be
merged into kvm-x86 tree?

While I not sure if the same developer can give both Tested-by and Reviewed-by ...

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


Thank you very much!

Dongli Zhang

On 10/20/23 00:45, Dongli Zhang wrote:
> 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