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Re: [Qemu-devel] [PATCH 2/5] trace: [all] Add "guest_vmem" event

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH 2/5] trace: [all] Add "guest_vmem" event
Date: Sun, 20 Mar 2016 19:59:55 +0000

On 20 March 2016 at 18:09, Lluís Vilanova <address@hidden> wrote:
> Oh! Yes, I seem to remember that code path now, I checked it a
> really long time ago. I was assuming that whenever this event is
> enabled at compile time, I would have to modify QEMU's TLB to store
> the guest physical address (then used by the tracing event).

I guess we could maybe put that into the iotlb. You definitely
don't want it in the CPUTLBEntry struct as that one is space
critical for performance. (If you're really lucky you can
reconstruct the physaddr from the iotlb addr field but I suspect
you can't.)

Once you've decided to take the hit of keeping the paddr in the
TLB it's probably faster to just unconditionally store it rather
than doing a "store if trace event X enabled" test.

PS: you probably also want to be able to trace the MemTxAttrs
(which tells you whether you have an S or NS access on ARM,
among other things).

-- PMM

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