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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Tue, 05 Jul 2016 14:11:05 +0100
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Subject: Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the
 hot-path
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Emilio G. Cota" <cota@braap.org>, mttcg@listserver.greensocs.com, peter.maydell@linaro.org, claudio.fontana@huawei.com, Peter Crosthwaite <crosthwaite.peter@gmail.com>, jan.kiszka@siemens.com, mark.burton@greensocs.com, a.rigo@virtualopensystems.com, qemu-devel@nongnu.org, serge.fdrv@gmail.com, bobby.prani@gmail.com, rth@twiddle.net, fred.konrad@greensocs.com


Paolo Bonzini <pbonzini@redhat.com> writes:

> On 05/07/2016 13:14, Alex Bennée wrote:
>> /*
>>  * Patch the last TB with a jump to the current TB.
>>  *
>>  * Modification of the TB has to be protected with tb_lock which can
>>  * either be already held or taken here.
>>  */
>> static inline void maybe_patch_last_tb(CPUState *cpu,
>>                                        TranslationBlock *tb,
>>                                        TranslationBlock **last_tb,
>>                                        int tb_exit,
>>                                        bool locked)
>> {
>>     if (cpu->tb_flushed) {
>>         /* Ensure that no TB jump will be modified as the
>>          * translation buffer has been flushed.
>>          */
>>         *last_tb = NULL;
>>         cpu->tb_flushed = false;
>>     }
>> #ifndef CONFIG_USER_ONLY
>>     /* We don't take care of direct jumps when address mapping changes in
>>      * system emulation. So it's not safe to make a direct jump to a TB
>>      * spanning two pages because the mapping for the second page can change.
>>      */
>>     if (tb->page_addr[1] != -1) {
>>         *last_tb = NULL;
>>     }
>> #endif
>>     /* See if we can patch the calling TB. */
>>     if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
>>         if (!locked) {
>>             tb_lock();
>>         }
>>         tb_add_jump(*last_tb, tb_exit, tb);
>>         if (!locked) {
>>             tb_unlock();
>>         }
>>     }
>> }
>
> Why not add tb_lock_recursive() and tb_lock_reset()?

I thought we didn't like having recursive locking? I agree it would make
things a little neater though.

--
Alex Bennée