Re: [Qemu-devel] [PATCH 00/13] instrument: Add basic event instrumentation

[Peter Maydell]

On 27 July 2017 at 16:21, Daniel P. Berrange <address@hidden> wrote:
> On Thu, Jul 27, 2017 at 11:54:29AM +0100, Peter Maydell wrote:
>> That said, yes, I was going to ask if we could do this via
>> leveraging the tracepoint infrastructure and whatever scripting
>> facilities it provides. Are there any good worked examples of
>> this sort of thing? Can you do it as an ordinary non-root user?
>
> Do you have a particular thing you'd like to see an example of ?
>
> To dynamically probe a function which doesn't have a tracepoint
> defined you can do:
>
> probe process("/usr/bin/qemu-x86_64").function("helper_syscall") {
>   printf("syscall stasrt\n")
> }
>
> but getting access to the function args is not as easy as with
> pre-defined tracepoints.

How do I go about actually running that script? What I
have in mind by "worked example" is something like a blog
post that says "ok, here's a problem, we want to find out
what QEMU is doing in situation X, here's how you do this
with $TRACING_THINGY" and generally steps you through how
it works assuming you know nothing at all about whatever
the tracing facility you're using is.

> You can't typically run this as root,

Do you mean "non-root" ?

> however, I don't think that's a
> huge issue, because most QEMU deployments are not running as your own
> user account anyway, so you can't directly interact with them no
> matter what.

It is important, because almost all uses of TCG QEMU are
running it from the command line as non-root normal users,
especially if they're trying to debug what's going on with a
guest binary. So any tracing solution for this kind of usecase
must work without requiring root access, I think.

I think the users for this are pretty much completely
distinct from anybody who would use the term "deployment"
for their usage of QEMU :-)

thanks
-- PMM