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From: Sergey Fedorov <serge.fdrv@gmail.com>
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Date: Fri, 3 Jun 2016 22:52:07 +0300
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Subject: Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for
 memory barrier
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To: Pranith Kumar <bobby.prani@gmail.com>, Richard Henderson <rth@twiddle.net>
Cc: "open list:All patches CC here" <qemu-devel@nongnu.org>, serge.fdrv@linaro.org, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>

On 03/06/16 21:27, Pranith Kumar wrote:
> On Thu, Jun 2, 2016 at 5:18 PM, Richard Henderson <rth@twiddle.net> wrote:
>> >
>> > Hum.  That does seem helpful-ish.  But I'm not certain how helpful it is to
>> > complicate the helper functions even further.
>> >
>> > What if we have tcg_canonicalize_memop (or some such) split off the barriers
>> > into separate opcodes.  E.g.
>> >
>> > MO_BAR_LD_B = 32        // prevent earlier loads from crossing current op
>> > MO_BAR_ST_B = 64        // prevent earlier stores from crossing current op
>> > MO_BAR_LD_A = 128       // prevent later loads from crossing current op
>> > MO_BAR_ST_A = 256       // prevent later stores from crossing current op
>> > MO_BAR_LDST_B = MO_BAR_LD_B | MO_BAR_ST_B
>> > MO_BAR_LDST_A = MO_BAR_LD_A | MO_BAR_ST_A
>> > MO_BAR_MASK = MO_BAR_LDST_B | MO_BAR_LDST_A
>> >
>> > // Match Sparc MEMBAR as the most flexible host.
>> > TCG_BAR_LD_LD = 1       // #LoadLoad barrier
>> > TCG_BAR_ST_LD = 2       // #StoreLoad barrier
>> > TCG_BAR_LD_ST = 4       // #LoadStore barrier
>> > TCG_BAR_ST_ST = 8       // #StoreStore barrier
>> > TCG_BAR_SYNC  = 64      // SEQ_CST barrier
> I really like this format. I would also like to add to the frontend:
>
> MO_BAR_ACQUIRE
> MO_BAR_RELEASE
>
> and the following to the backend:
>
> TCG_BAR_ACQUIRE
> TCG_BAR_RELEASE
>
> since these are one-way barriers and the previous barrier types do not
> cover them.
>

Well, my last comment about acq/rel barriers should really go here
[Message-Id: 5751DF2D.5040709@gmail.com]

Regards,
Sergey