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

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Date:   Fri, 15 Dec 2017 11:10:53 +0100
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
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 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?

Thanks,
-Christoffer