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From: Chen Gang <chengang@emindsoft.com.cn>
Message-ID: <56977A39.8060405@emindsoft.com.cn>
Date: Thu, 14 Jan 2016 18:36:41 +0800
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Subject: Re: [Qemu-devel] [PATCH] translate-all.c: Remove writable
 protection feature for tb_alloc_page()
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>, QEMU Developers <qemu-devel@nongnu.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On 2016年01月14日 18:30, Peter Maydell wrote:
> On 14 January 2016 at 10:26, Chen Gang <chengang@emindsoft.com.cn> wrote:
>> On 2016年01月14日 18:05, Peter Maydell wrote:
>>> If we don't mark the page as non-writeable when we generate a TB
>>> from it, how do we detect when guest code later writes to that
>>> page (which means we need to invalidate the TB) ?
>>>
>>
>> For me, what you said above sounds reasonable, at present, that's really
>> valuable to me :-)
>>
>> I guess, you also mean: our qemu will catch the host page fault signal
>> and invalidate the TB.
> 
> Yes, this is how it works for user-mode. (For softmmu we can catch
> writes and send them via the slow path which does the check for
> whether TBs need to be invalidated; for linux-user we have no
> emulated MMU so we must rely on the host kernel sending us the
> SIGSEGV.) The bit of code that does this is at the top of
> handle_cpu_signal():
> 
>     if (is_write && h2g_valid(address)
>         && page_unprotect(h2g(address), pc, puc)) {
>         return 1;
>     }
> 

OK, thank you very much!  :-)


Thanks.
-- 
Chen Gang (陈刚)

Open, share, and attitude like air, water, and life which God blessed