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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Tue, 26 Apr 2016 07:35:23 +0100
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Subject: Re: [Qemu-devel] [RFC v3] translate-all: protect code_gen_buffer
 with RCU
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To: Richard Henderson <rth@twiddle.net>
Cc: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Sergey Fedorov <serge.fdrv@gmail.com>


Richard Henderson <rth@twiddle.net> writes:

> On 04/25/2016 04:46 PM, Emilio G. Cota wrote:
>> +    /*
>> +     * write the prologue into buf2. This is safe because we'll later call
>> +     * tcg_prologue_init on buf1, from which we'll start execution.
>> +     */
>> +    tcg_ctx.code_gen_buffer = code_gen_buf2;
>> +    tcg_prologue_init(&tcg_ctx);
>> +
>
> Ah, no.  Write only one prologue, not one per buffer.
>
> If they're sufficiently close (i.e. one allocation under the max size),
> then the same one can be used for both halves.
>
> The global variables that you didn't see in this revision are:
>
> aarch64/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> arm/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> i386/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> ia64/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> ia64/tcg-target.inc.c:    tcg_insn_unit *thunks[8] = { };
> mips/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> ppc/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> s390/tcg-target.inc.c:static tcg_insn_unit *tb_ret_addr;
> sparc/tcg-target.inc.c:static tcg_insn_unit *qemu_ld_trampoline[16];
> sparc/tcg-target.inc.c:static tcg_insn_unit *qemu_st_trampoline[16];

Aside from the existing code structure is there any reason to have only
one prologue? It doesn't seem to be a large amount of code and in the
case of having smaller translation regions I would posit having a
"local" prologue/epilogue would make the jumps cheaper.

>
>
> r~


--
Alex Bennée