Re: [Qemu-devel] [PATCH 0/5] trace: Add events for vCPU memory accesses

[Peter Maydell]

On 22 March 2016 at 14:02, Lluís Vilanova <address@hidden> wrote:
> Stefan Hajnoczi writes:
>
>> On Wed, Mar 16, 2016 at 03:10:01PM +0000, Peter Maydell wrote:
>>> The first two patches which add TCG guest data access tracing look
>>> OK to me, but I'm much less sure about the last three which are
>>> adding tracing into linux-user syscall emulation. I'm not sure
>>> that lock_user is the right place to put that tracepoint.
>
>> Any thoughts on this, Lluís?
>
> Mmmm, I was struggling to find a place to easily add the tracing events
> whenever the syscall emulation code accesses guest memory.
>
> The lock_user function is used precisely for that, but it can be a bit
> heavy-handed as to what memory is actually read/written, since it only marks 
> the
> "potential" ability of doing so (it's a sort of acquire/release interface that
> differentiates between read and write acquires).

Well, that's why I'm not certain about it. I would prefer us to
not trace the accesses rather than put a trace in a wrong
position.

thanks
-- PMM