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From: Peter Maydell <peter.maydell@linaro.org>
Date: Tue, 29 Mar 2016 17:26:04 +0100
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Subject: Re: [Qemu-devel] [Qemu-arm] [PATCH 1/8] tcg: Clean up direct block
	chaining data fields
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To: Sergey Fedorov <serge.fdrv@gmail.com>
Cc: Sergey Fedorov <sergey.fedorov@linaro.org>, Stefan Weil <sw@weilnetz.de>, Claudio Fontana <claudio.fontana@huawei.com>, QEMU Developers <qemu-devel@nongnu.org>, Alexander Graf <agraf@suse.de>, Blue Swirl <blauwirbel@gmail.com>, qemu-arm <qemu-arm@nongnu.org>, "Vassili Karpov (malc)" <av1474@comtv.ru>, Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>, Aurelien Jarno <aurelien@aurel32.net>, Richard Henderson <rth@twiddle.net>

On 29 March 2016 at 09:31, Sergey Fedorov <serge.fdrv@gmail.com> wrote:
> On 29/03/16 01:12, Richard Henderson wrote:
>> On 03/24/2016 08:11 AM, Paolo Bonzini wrote:
>>> There is also a case where a TB jumps to itself; it then appears twice
>>> in the list with different values in the low bits, such as this:
>>>
>>>      tb->jmp_list_first = tb | 0;
>>>       .--------------------'   |
>>>       |                .-------'
>>>      tb->jmp_list_next[0] = tb | 2;
>>
>> Of course, it begs the question of why TB would be in its own list,
>> even if it does jump to itself.  We only need the points-to list in
>> order to invalidate a TB and unlink it.  But if TB is being
>> invalidated, we don't need to reset the jump within TB itself.
>
> If we're going to move tb_phys_invalidate() outside of tb_lock, we
> probably need to reset all jumps to the TB, even if it jumps to itself,
> so that it eventually finish its execution.

This is likely also the historical reason for the current code --
originally we handled requesting a CPU exit by unlinking the TB,
so you needed to be able to detach jumps-to-self (these days we do
it by checking a flag at the start of each TB).

thanks
-- PMM