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Re: [Qemu-devel] [RFC v2 04/11] tcg: comment on which functions have to

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC v2 04/11] tcg: comment on which functions have to be called with tb_lock held
Date: Wed, 11 May 2016 15:46:12 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 11/05/2016 15:36, Sergey Fedorov wrote:
> On 11/05/16 15:58, Paolo Bonzini wrote:
>> On 06/05/2016 20:22, Sergey Fedorov wrote:
>>> However, there's no sensible description of what is protected by tb_lock
>>> and mmap_lock. I think we need to have a clear documented description of
>>> the TCG locking scheme in order to be sure we do right things in MTTCG.
>> I think there was such a patch somewhere, but: tb_lock basically
>> protects tcg_ctx, while mmap_lock protects the user-mode emulation page
>> table (the equivalent for system emulation is the memory map which is
>> protected by the BQL).  Furthermore, mmap_lock must be taken outside
>> tb_lock.
> What's a user-mode emulation page table? 'l1_map'?

Yes.  It's used beyond TCG in user-mode emulation.

> It is used by system
> emulation to keep track of TBs per page and 'code_bitmap'. Shouldn't it
> be protected with 'mmap_lock' in system emulation?

tb_lock is used instead because it's taken everywhere system emulation
uses l1_map; so tb_lock is protecting l1_map too in system emulation.

As mentioned above, user-mode emulation uses l1_map in linux-user/mmap.c
via page_{get,set}_flags, which I guess is why the lock is separate.
None of us was involved in the original multi-threaded linux-user work,
we're reverse engineering it just like you. :)



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