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Re: [Qemu-devel] [PATCH 1/2] [TEST] aarch64: Use pmuserenr_el0 register


From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 1/2] [TEST] aarch64: Use pmuserenr_el0 register for instrumentation
Date: Wed, 28 Jun 2017 10:27:58 +0100

On 28 June 2017 at 06:00, Pranith Kumar <address@hidden> wrote:
> We need a way for the benchmark running in the guest to indicate us to
> start/stop our instrumentation. On x86, we could use the 'cpuid'
> instruction along with an appropriately populated 'eax'
> register. However, no such dummy instruction exists for aarch64. So
> we modify the permission bits for 'pmuserenr_el0' register and tap
> that to instrument the guest code.

Why not just define a QEMU-specific system register in the
reserved-for-implementation-defined-registers encoding range,
rather than modifying the behaviour of an architecturally
defined register?

(Though overall I feel we should be able to do better as
a mechanism.)

thanks
-- PMM



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