linux-kernel - Re: [PATCH] KVM: PPC: Defer vtime accounting 'til after IRQ handling

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Message-ID: <1bc32339-b786-99bd-d49c-ad19ac7ce29b@redhat.com>
Date:   Thu, 7 Oct 2021 15:45:13 +0200
From:   Laurent Vivier <lvivier@...hat.com>
To:     Greg Kurz <groug@...d.org>
Cc:     kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH] KVM: PPC: Defer vtime accounting 'til after IRQ handling

On 06/10/2021 12:42, Greg Kurz wrote:
> On Wed,  6 Oct 2021 09:37:45 +0200
> Laurent Vivier <lvivier@...hat.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=2009312
>> 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@...il.com
>>
>> Signed-off-by: Laurent Vivier <lvivier@...hat.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@...gle.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