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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>; 
Cc: 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 'George Dunlap' 
<george.dunlap@xxxxxxxxxx>; 'Ian
Jackson' <ian.jackson@xxxxxxxxxxxxx>; 'Stefano Stabellini' 
<sstabellini@xxxxxxxxxx>; 'Wei Liu'
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
<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

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?

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?


Julien Grall



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