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From: Sergey Fedorov <serge.fdrv@gmail.com>
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Date: Mon, 11 Jul 2016 16:06:09 +0300
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Subject: Re: [Qemu-devel] [PATCH 3/3] tcg: Avoid bouncing tb_lock between
 tb_gen_code() and tb_add_jump()
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: mttcg@greensocs.com, peter maydell <peter.maydell@linaro.org>, claudio fontana <claudio.fontana@huawei.com>, Sergey Fedorov <sergey.fedorov@linaro.org>, patches@linaro.org, jan kiszka <jan.kiszka@siemens.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, mark burton <mark.burton@greensocs.com>, qemu-devel@nongnu.org, a rigo <a.rigo@virtualopensystems.com>, cota@braap.org, bobby prani <bobby.prani@gmail.com>, rth@twiddle.net, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, fred konrad <fred.konrad@greensocs.com>

On 08/07/16 23:52, Paolo Bonzini wrote:
>
> On 08/07/2016 22:24, Sergey Fedorov wrote:
>> I remember, I've just found that we discussed it in this thread:
>>
>> http://thread.gmane.org/gmane.comp.emulators.qemu/401723/focus=406852
>>
>> I was thinking of just doing 'tb_jmp_cache' lookup out of the lock, not
>> tb_find_physical(). Now thanks to QHT, we could do tb_find_physical()
>> out of the lock, too. This changes things.
> But in my patch ("tcg: move tb_find_fast outside the tb_lock critical
> section", which originally was written by Fred---most of my contribution
> was getting the invalidation right, not the lock-free lookup)
> tb_find_physical was also done out of the lock.  It was then retried
> inside the lock, if it failed.
>
> This is why I needed to fail all concurrent lookups as the first step in
> the invalidation.
>
> Emilio's QHT resulted in a rewrite of tb_find_physical, but the basic
> concepts are the same.

That could work, I think, if we re-check under tb_lock whether the TB is
still valid before adding a direct jump to it.

Kind regards,
Sergey