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Date: Tue, 6 Jun 2017 12:25:08 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20170606162508.GB6109@flamenco>
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Subject: Re: [Qemu-devel] [PATCH v2 3/3] tcg: allocate TB structs before the
 corresponding translated code
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To: Richard Henderson <rth@twiddle.net>
Cc: qemu-devel@nongnu.org, alex.bennee@linaro.org, Peter Maydell <peter.maydell@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Pranith Kumar <bobby.prani@gmail.com>

On Tue, Jun 06, 2017 at 01:24:11 -0700, Richard Henderson wrote:
> On 06/05/2017 03:49 PM, Emilio G. Cota wrote:
> >+TranslationBlock *tcg_tb_alloc(TCGContext *s)
> >+{
> >+    void *aligned;
> >+
> >+    aligned = (void *)ROUND_UP((uintptr_t)s->code_gen_ptr, QEMU_CACHELINE_SIZE);
> >+    if (unlikely(aligned + sizeof(TranslationBlock) > s->code_gen_highwater)) {
> >+        return NULL;
> >+    }
> >+    s->code_gen_ptr += aligned - s->code_gen_ptr + sizeof(TranslationBlock);
> >+    return aligned;
> 
> We don't really need the 2/3 patch.  We don't gain anything by telling the
> compiler that the structure is more aligned than it needs to be.

The compile-time requirement is for the compiler to pad the structs
appropriately; this is critical to avoid false sharing when allocating
arrays of structs like those test programs do.

> We can query the line size at runtime, as suggested by Pranith, and use that
> for the alignment here.  Which means that the binary isn't tied to a
> particular cpu implementation, which is clearly preferable for
> distributions.

For this particular case we can get away without padding the structs if
we're OK with having the end of a TB struct immediately followed
by its translated code, instead of having that code on the following
cache line.

		E.