linux-kernel - Re: [PATCH] KVM: arm/arm64: don't set vtimer->cnt_ctl in kvm_arch_timer

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Message-ID: <83d52c86-0c09-0a18-34e3-55eb213f8083@arm.com>
Date:   Fri, 15 Dec 2017 10:33:48 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Christoffer Dall <christoffer.dall@...aro.org>
Cc:     Jia He <hejianet@...il.com>, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Jia He <jia.he@...-semitech.com>
Subject: Re: [PATCH] KVM: arm/arm64: don't set vtimer->cnt_ctl in
 kvm_arch_timer_handler

On 15/12/17 10:10, Christoffer Dall wrote:
> On Fri, Dec 15, 2017 at 09:09:05AM +0000, Marc Zyngier wrote:
>> On 15/12/17 02:27, Jia He wrote:
>>>
>>>
>>
>> [...]
>>
>>>> @@ -367,6 +368,7 @@ static void vtimer_save_state(struct kvm_vcpu *vcpu)
>>>>   
>>>>   	/* Disable the virtual timer */
>>>>   	write_sysreg_el0(0, cntv_ctl);
>>>> +	isb();
>>> My only concern is whether this isb() is required here?
>>> Sorryif this is a stupid question.I understand little about arm arch 
>>> memory barrier. But seems isb will flush all the instruction prefetch.Do 
>>> you think if an timer interrupt irq arrives, arm will use the previous 
>>> instruction prefetch?
>>
>>
>> This barrier has little to do with prefetch. It just guarantees that the
>> code after the isb() is now running with a disabled virtual timer.
>> Otherwise, a CPU can freely reorder the write_sysreg() until the next
>> context synchronization event.
>>
>> An interrupt coming between the write and the barrier will also act as a
>> context synchronization event. For more details, see the ARMv8 ARM (the
>> glossary has a section on the concept).
>>
> 
> So since an ISB doesn't guarantee that the timer will actually be
> disabled, is it a waste of energy to have it, or should we keep it as a
> best effort kind of thing?

nit: the ISB does offer that guarantee. It is just that the guarantee
doesn't extend to an interrupt that has already been signalled.

The main issue I have with not having an ISB is that it makes it harder
to think of when the disabling actually happens. The disabling could be
delayed for a very long time, and would make things harder to debug if
they were going wrong.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...