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Re: [Qemu-devel] [RFC PATCH V6 15/18] cpu: introduce tlb_flush*_all.

From: Mark Burton
Subject: Re: [Qemu-devel] [RFC PATCH V6 15/18] cpu: introduce tlb_flush*_all.
Date: Mon, 6 Jul 2015 16:29:09 +0200

Paolo, Alex, Alexander,

Talking to Fred after the call about ways of avoiding the ‘stop the world’ (or 
rather ‘sync the world’) - we already discussed this on this thread.
One thing that would be very helpful would be some test cases around this. We 
could then use Fred’s code to check some of the possible solutions out….

I’m not sure if there is wiggle room in Peter’s statement below. Can the TLB 
operation be completed on one core, but not ‘seen’ by other cores until they 
hit an exit…..?



> On 26 Jun 2015, at 18:30, Frederic Konrad <address@hidden> wrote:
> On 26/06/2015 18:08, Peter Maydell wrote:
>> On 26 June 2015 at 17:01, Paolo Bonzini <address@hidden> wrote:
>>> On 26/06/2015 17:54, Frederic Konrad wrote:
>>>> So what happen is:
>>>> An arm instruction want to clear tlb of all VCPUs eg: IS version of
>>>> The VCPU which execute the TLBIALL_IS can't flush tlb of other VCPU.
>>>> It will just ask all VCPU thread to exit and to do tlb_flush hence the
>>>> async_work.
>>>> Maybe the big issue might be memory barrier instruction here which I didn't
>>>> checked.
>>> Yeah, ISTR that in some cases you have to wait for other CPUs to
>>> invalidate the TLB before proceeding.  Maybe it's only when you have a
>>> dmb instruction, but it's probably simpler for QEMU to always do it
>>> synchronously.
>> Yeah, the ARM architectural requirement here is that the TLB
>> operation is complete after a DSB instruction executes. (True for
>> any TLB op, not just the all-CPUs ones). NB that we also call
>> tlb_flush() from target-arm/ code for some things like "we just
>> updated a system register"; some of those have "must take effect
>> immediately" semantics.
>> In any case, for generic code we have to also consider the
>> semantics of non-ARM guests...
>> thanks
>> -- PMM
> Yes this is not the case as I implemented it.
> The rest of the TB will be executed before the tlb_flush work really happen.
> The old version did this, was slow and was a mess (if two VCPUs want to 
> tlb_flush
> at the same time and an other tlb_flush_page.. it becomes tricky..)
> I think it's not really terrible if the other VCPU execute some stuff before 
> doing the
> tlb_flush.? So the solution would be only to cut the TranslationBlock after 
> instruction
> which require a tlb_flush?
> Thanks,
> Fred

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