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

From: Lluís Vilanova
Subject: Re: [Qemu-devel] [PATCH 2/5] trace: [all] Add "guest_vmem" event
Date: Mon, 21 Mar 2016 17:51:52 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Peter Maydell writes:

> 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.

I meant to make the check at compile time, since we have defines to check which
events are enabled/disabled in trace-events.

> 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).

I'll keep these in mind for a separate series with extended memory info.


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