Re: [Qemu-devel] [PATCH 02/10] compiler.h: add QEMU_CACHELINE + QEMU_ALIGN() + QEMU_CACHELINE_ALIGNED

[Emilio G. Cota]

On Tue, Apr 05, 2016 at 08:57:45 +0100, Peter Maydell wrote:
> On 5 April 2016 at 06:30, Emilio G. Cota <address@hidden> 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