From: Laurent Vivier <lvivier@redhat.com>
To: Greg Kurz <groug@kaod.org>
Cc: kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Michael Ellerman <mpe@ellerman.id.au>,
linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
Paul Mackerras <paulus@ozlabs.org>
Subject: Re: [PATCH] KVM: PPC: Defer vtime accounting 'til after IRQ handling
Date: Thu, 07 Oct 2021 13:45:13 +0000 [thread overview]
Message-ID: <1bc32339-b786-99bd-d49c-ad19ac7ce29b@redhat.com> (raw)
In-Reply-To: <20211006124204.4741bb5c@bahia.huguette>
On 06/10/2021 12:42, Greg Kurz wrote:
> On Wed, 6 Oct 2021 09:37:45 +0200
> Laurent Vivier <lvivier@redhat.com> wrote:
>
>> Commit 61bd0f66ff92 has moved guest_enter() out of the interrupt
>> protected area to be able to have updated tick counters, but
>> commit 112665286d08 moved back to this area to avoid wrong
>> context warning (or worse).
>>
>> None of them are correct, to fix the problem port to POWER
>> the x86 fix 160457140187 ("KVM: x86: Defer vtime accounting 'til
>> after IRQ handling"):
>>
>> "Defer the call to account guest time until after servicing any IRQ(s)
>> that happened in the guest or immediately after VM-Exit. Tick-based
>> accounting of vCPU time relies on PF_VCPU being set when the tick IRQ
>> handler runs, and IRQs are blocked throughout the main sequence of
>> vcpu_enter_guest(), including the call into vendor code to actually
>> enter and exit the guest."
>>
>> Link: https://bugzilla.redhat.com/show_bug.cgi?id 09312
>> Fixes: 61bd0f66ff92 ("KVM: PPC: Book3S HV: Fix guest time accounting with VIRT_CPU_ACCOUNTING_GEN")
>
> This patch was merged in linux 4.16 and thus is on the 4.16.y
> stable branch and it was backported to stable 4.14.y. It would
> make sense to Cc: stable # v4.14 also, but...
>
>> Fixes: 112665286d08 ("KVM: PPC: Book3S HV: Context tracking exit guest context before enabling irqs")
>
> ... this one, which was merged in linux 5.12, was never backported
> anywhere because it wasn't considered as a fix, as commented here:
>
> https://lore.kernel.org/linuxppc-dev/1610793296.fjhomer31g.astroid@bobo.none/
>
> AFAICT commit 61bd0f66ff92 was never mentioned anywhere in a bug. The
> first Fixes: tag thus looks a bit misleading. I'd personally drop it
> and Cc: stable # v5.12.
>
Ok, I update the comment.
>> Cc: npiggin@gmail.com
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>> arch/powerpc/kvm/book3s_hv.c | 10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
>> index 2acb1c96cfaf..43e1ce853785 100644
>> --- a/arch/powerpc/kvm/book3s_hv.c
>> +++ b/arch/powerpc/kvm/book3s_hv.c
>> @@ -3695,6 +3695,8 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
>>
>> srcu_read_unlock(&vc->kvm->srcu, srcu_idx);
>>
>> + context_tracking_guest_exit();
>> +
>> set_irq_happened(trap);
>>
>> spin_lock(&vc->lock);
>> @@ -3726,9 +3728,8 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
>>
>> kvmppc_set_host_core(pcpu);
>>
>> - guest_exit_irqoff();
>> -
>
>
> Change looks ok but it might be a bit confusing for the
> occasional reader that guest_enter_irqoff() isn't matched
> by a guest_exit_irqoff().
>
>> local_irq_enable();
>> + vtime_account_guest_exit();
>>
>
> Maybe add a comment like in x86 ?
>
done
>> /* Let secondaries go back to the offline loop */
>> for (i = 0; i < controlled_threads; ++i) {
>> @@ -4506,13 +4507,14 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
>>
>> srcu_read_unlock(&kvm->srcu, srcu_idx);
>>
>> + context_tracking_guest_exit();
>> +
>> set_irq_happened(trap);
>>
>> kvmppc_set_host_core(pcpu);
>>
>> - guest_exit_irqoff();
>> -
>> local_irq_enable();
>> + vtime_account_guest_exit();
>>
>> cpumask_clear_cpu(pcpu, &kvm->arch.cpu_in_guest);
>>
>
> Same remarks. FWIW a followup was immediately added to x86 to
> encapsulate the enter/exit logic in common helpers :
>
> ommit bc908e091b3264672889162733020048901021fb
> Author: Sean Christopherson <seanjc@google.com>
> Date: Tue May 4 17:27:35 2021 -0700
>
> KVM: x86: Consolidate guest enter/exit logic to common helpers
>
> This makes the code nicer. Maybe do something similar for POWER ?
I don't like to modify kernel code when it's not needed. I just want to fix a bug, if
someone wants this nicer I let this to him...
Thanks,
Laurent
prev parent reply other threads:[~2021-10-07 13:45 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-06 7:37 [PATCH] KVM: PPC: Defer vtime accounting 'til after IRQ handling Laurent Vivier
2021-10-06 10:42 ` Greg Kurz
2021-10-07 13:45 ` Laurent Vivier [this message]
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