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

[Frederic Konrad <fred.konrad@greensocs.com>]

On 26/06/2015 18:08, Peter Maydell wrote:
> On 26 June 2015 at 17:01, Paolo Bonzini <pbonzini@redhat.com> 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
>>> TLBIALL.
>>> 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
Hi,

About that we plan to:
   * make tlb_flush work sync and not async (in case of a tlb_flush_all).
   * break the TranslationBlock after a DSB.

In this case when we have a tlb_flush_all, all VCPU's threads will exit 
and wait for all
VCPUs to be out of cpu_exec before doing the flush. Then they won't be 
able to
enter cpu_exec until any flush remains. So in case of a DSB, if there is 
any pending
tlb_flush it won't be able to enter cpu_exec until it is done so we have 
the right
behaviour I think.

The obscur part is: what should happen if CPU A flush it's tlb itself 
and CPU B does
a DSB? I'm not sure if this is really a problem if CPU A didn't finish 
it's TLB operation
as the DSB might have happened before the flush operation?

Do that makes sense?

Thanks,
Fred