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Date: Tue, 5 Apr 2016 13:24:53 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160405172453.GA5796@flamenco>
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Subject: Re: [Qemu-devel] [PATCH 02/10] compiler.h: add QEMU_CACHELINE +
 QEMU_ALIGN() + QEMU_CACHELINE_ALIGNED
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To: Peter Maydell <peter.maydell@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>
Cc: MTTCG Devel <mttcg@listserver.greensocs.com>, MTTCG Devel <mttcg@greensocs.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, QEMU Developers <qemu-devel@nongnu.org>, Sergey Fedorov <serge.fdrv@gmail.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Richard Henderson <rth@twiddle.net>

On Tue, Apr 05, 2016 at 08:57:45 +0100, Peter Maydell wrote:
> On 5 April 2016 at 06:30, Emilio G. Cota <cota@braap.org> wrote:
> > +#define QEMU_CACHELINE (64)
> 
> Why 64? Does anything bad happen if the host's cache line
> size turns out to be greater than the value here ?

Defining a number lower than the host's cache line could result
in some false sharing. No big deal for most workloads.

Defining a number greater than the host's cache might result in
wasted memory, which we really should avoid.

I used 64 because it is common on x86, which I assume is what most
developers develop on.

On Tue, Apr 05, 2016 at 17:29:05 +0200, Paolo Bonzini wrote:
> But the size of data structures cannot change at run-time.  A bigger
> worry is cross-compilation.
> 
> Linux has these defaults:
> 
> 	Alpha	32 or 64
> 	ARM	configurable, 64 for ARMv7
> 	AArch64	128
> 	PPC     128
> 	s390	256
> 	x86	configurable, default 64
> 
> which should be easy to copy in QEMU too.

So how about this:
we add these defaults, and also add an optional --configure
parameter to override said defaults.

Otherwise I'd just stick to 64.

Thanks,

		Emilio