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Re: [Qemu-devel] [PATCH 00/13] ARM: Add LPAE support

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 00/13] ARM: Add LPAE support
Date: Wed, 4 Jul 2012 10:47:45 +0100

On 28 June 2012 15:35, Peter Maydell <address@hidden> wrote:
> It's possible that making target_phys_addr_t 64 bits for
> all ARM cores might be a slight slowdown (though only in
> system mode, and not in the fast path of a TLB hit for a
> RAM access). Benchmarking this is on my todo list, but I wanted
> to put the patchset out for review to start with.

I've now done some (slightly quick-and-dirty) benchmarking.
Test 1 was a vexpress-a9 ARM guest booting Linux (linaro 1108 nano)
to root login and immediate shutdown. There was basically no
difference in time between with-64-bit-physaddrs and without,
on either i386 or x86 host (ie the ~0.5s between run variation
on a 45s test was more than the 64-vs-32 variation).

Test 2 involved running a popular embedded benchmark in system
mode (again, vexpress-a9 linaro 1108 nano guest). I compiled
the benchmark -O3 -DNDEBUG -mfloat-abi=soft, which means we're
looking at integer code running inside QEMU rather than testing
the speed of the softfloat fpu emulation. A few of the benchmarks
actually claimed to run quicker with 64 bit physaddrs, most were
within 0.5% or so of the 32 bit runs. Again, about the same for
both i386 and x86-64 guest.

So overall my best estimate is that this change is a measurable
but pretty insignificant slowdown (~0.5% or so). I think that's
worth taking to allow LPAE guests to get at more than 4GB of RAM.

-- PMM

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