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

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Message-ID: <519f0e33-4419-68be-32b4-11bb5e19cf17@arm.com>
Date:   Fri, 15 Dec 2017 09:09:05 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Jia He <hejianet@...il.com>,
        Christoffer Dall <christoffer.dall@...aro.org>
Cc:     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 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).

Thanks,

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