RE: [PATCH 0/3] xenoprof: XSA-313 follow-up

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 24 April 2020 11:35
> To: paul@xxxxxxx; 'Jan Beulich' <jbeulich@xxxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 'George Dunlap' 
> <george.dunlap@xxxxxxxxxx>; 'Ian
> Jackson' <ian.jackson@xxxxxxxxxxxxx>; 'Stefano Stabellini' 
> <sstabellini@xxxxxxxxxx>; 'Wei Liu'
> <wl@xxxxxxx>
> Subject: Re: [PATCH 0/3] xenoprof: XSA-313 follow-up
> 
> Hi Paul,
> 
> On 20/04/2020 16:01, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Julien Grall <julien@xxxxxxx>
> >> Sent: 20 April 2020 15:15
> >> To: paul@xxxxxxx; 'Jan Beulich' <jbeulich@xxxxxxxx>; 
> >> xen-devel@xxxxxxxxxxxxxxxxxxxx
> >> Cc: 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 'George Dunlap' 
> >> <george.dunlap@xxxxxxxxxx>; 'Ian
> >> Jackson' <ian.jackson@xxxxxxxxxxxxx>; 'Stefano Stabellini' 
> >> <sstabellini@xxxxxxxxxx>; 'Wei Liu'
> >> <wl@xxxxxxx>
> >> Subject: Re: [PATCH 0/3] xenoprof: XSA-313 follow-up
> >>
> >> Hi Paul,
> >>
> >> On 15/04/2020 09:50, Paul Durrant wrote:
> >>>> -----Original Message-----
> >>>> From: Jan Beulich <jbeulich@xxxxxxxx>
> >>>> Sent: 15 April 2020 09:45
> >>>> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> >>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; George Dunlap 
> >>>> <george.dunlap@xxxxxxxxxx>; Ian
> >> Jackson
> >>>> <ian.jackson@xxxxxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Stefano 
> >>>> Stabellini
> >>>> <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Paul Durrant 
> >>>> <paul@xxxxxxx>
> >>>> Subject: [PATCH 0/3] xenoprof: XSA-313 follow-up
> >>>>
> >>>> Patch 1 was considered to become part of the XSA, but it was then
> >>>> decided against. The other two are a little bit of cleanup, albeit
> >>>> there's certainly far more room for tidying. Yet then again Paul,
> >>>> in his mail from Mar 13, was asking whether we shouldn't drop
> >>>> xenoprof altogether, at which point cleaning up the code would be
> >>>> wasted effort.
> >>>>
> >>>
> >>> That's still my opinion. This is a large chunk of (only passively 
> >>> maintained) code which I think
> is
> >> of very limited value (since it relates to an old tool, and it only works 
> >> for PV domains).
> >>
> >> While there are no active user we are aware of, this is an example on
> >> how to implement a profiler backend with Xen. So I would agree with
> >> Andrew here.
> >>
> >> IIRC, the reason behind your request is it makes difficult for your
> >> xenheap work. Am I correct? If so, do you have a thread explaining the
> >> issues?
> >
> > After shared info and grant table, it is the only other occurrence of a 
> > xenheap page shared with a
> non-system domain. Also, it cannot be trivially replaced with an 'extra' 
> domheap page because its
> assignment changes. Thus a whole bunch of cleanup work that I was hoping to 
> do (largely in
> domain_relinquish_resources and free_domheap_pages) is either ruled out, or 
> would have to special-case
> this type of page.
> 
> My knowledge of xenoprof is very limited, so I might be wrong.
> 
>  From my understanding, a buffer can only be shared between two domains:
>     - When in passive mode, the buffer will be shared with the primary
> profiler (always the hwdom per the check in xenoprof_op_init()).
>     - When in active mode, the buffer will be shared with the domain
> requesting to be profiled.
> 
> Would it be possible to consider to have a separate buffer for the
> passive mode and active mode?

I think that may well work, yes.

> 
> > Also, I am unconvinced that PV guests are sufficiently common these days 
> > (apart from dom0) that
> profiling them is of any real use.
> 
> Even an HVM guest can't profile itself, I think it would be useful to
> have dom0 to profile an HVM guest. Are you suggesting this doesn't work?
> 

No. That may work for a PV dom0; I'll have to try it. So, yes, passive 
profiling may indeed still have value.

  Paul