Re: [Xen-devel] [PATCH 2/2] domain: use PGC_extra domheap page for shared_info

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 06 March 2020 17:17
> To: Paul Durrant <xadimgnik@xxxxxxxxx>
> Cc: sstabellini@xxxxxxxxxx; julien@xxxxxxx; Volodymyr_Babchuk@xxxxxxxx; 
> wl@xxxxxxx;
> konrad.wilk@xxxxxxxxxx; andrew.cooper3@xxxxxxxxxx; ian.jackson@xxxxxxxxxxxxx;
> george.dunlap@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; 'David Woodhouse' 
> <dwmw2@xxxxxxxxxxxxx>
> Subject: Re: [PATCH 2/2] domain: use PGC_extra domheap page for shared_info
> 
> On 06.03.2020 17:27, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> Sent: 06 March 2020 13:46
> >> To: Paul Durrant <xadimgnik@xxxxxxxxx>
> >> Cc: sstabellini@xxxxxxxxxx; julien@xxxxxxx; Volodymyr_Babchuk@xxxxxxxx; 
> >> wl@xxxxxxx;
> >> konrad.wilk@xxxxxxxxxx; andrew.cooper3@xxxxxxxxxx; 
> >> ian.jackson@xxxxxxxxxxxxx;
> >> george.dunlap@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; 'David 
> >> Woodhouse' <dwmw2@xxxxxxxxxxxxx>
> >> Subject: Re: [PATCH 2/2] domain: use PGC_extra domheap page for shared_info
> >>
> >> On 06.03.2020 14:41, Paul Durrant wrote:
> >>>> -----Original Message-----
> >>>> From: Jan Beulich <jbeulich@xxxxxxxx>
> >>>> Sent: 06 March 2020 13:36
> >>>> To: Paul Durrant <xadimgnik@xxxxxxxxx>
> >>>> Cc: sstabellini@xxxxxxxxxx; julien@xxxxxxx; Volodymyr_Babchuk@xxxxxxxx; 
> >>>> wl@xxxxxxx;
> >>>> konrad.wilk@xxxxxxxxxx; andrew.cooper3@xxxxxxxxxx; 
> >>>> ian.jackson@xxxxxxxxxxxxx;
> >>>> george.dunlap@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; 'David 
> >>>> Woodhouse' <dwmw2@xxxxxxxxxxxxx>
> >>>> Subject: Re: [PATCH 2/2] domain: use PGC_extra domheap page for 
> >>>> shared_info
> >>>>
> >>>> On 06.03.2020 14:26, Paul Durrant wrote:
> >>>>>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of 
> >>>>>> Jan Beulich
> >>>>>> Sent: 06 March 2020 13:24
> >>>>>>
> >>>>>> On 06.03.2020 14:13, Paul Durrant wrote:
> >>>>>>> My aim is to make the separation abundantly obvious by getting rid
> >>>>>>> of shared xenheap pages (for non-system domains at least) but I
> >>>>>>> can't do that before converting shared_info and grant shared/status
> >>>>>>> frames to domheap.
> >>>>>>
> >>>>>> Following David's various replies - instead of going this route of
> >>>>>> replacing the sharing of xenheap pages by different logic, the
> >>>>>> same ought to be achievable by making the backing allocations come
> >>>>>> from the correct pool?
> >>>>>>
> >>>>>
> >>>>> I still prefer the simplification of not having to clean up the
> >>>>> shared xenheap page list in domain_kill() so IMO it is still worth
> >>>>> it for that alone.
> >>>>
> >>>> I don't see anything very special with the cleaning up in
> >>>> domain_kill() / domain_relinquish_resources(). What I'd view as
> >>>> more desirable in this regard is the general fact of needing
> >>>> two lists. But you realize there's a downside to this as well?
> >>>> dump_pageframe_info() will reliably show _all_ Xen heap pages
> >>>> associated with a domain, but it will only ever show up to 10
> >>>> pages on ->page_list for a domain that's not already being
> >>>> cleaned up.
> >>>
> >>> That's not much of a downside though I don't think. The xenheap
> >>> (or PGC_extra domheap pages) are 'special' and so info about
> >>> them ought to be available via an alternate dump function anyway
> >>> (and if not already, it can be added).
> >>
> >> Whatever you'd add, the logic would need to either traverse the
> >> entire ->page_list (can take very long) or have/use out of band
> >> information where such pages may have a record (fragile).
> >>
> >
> > But the shared xenheap pages in question are only shared info, or
> > grant table, so their information can be dumped separately.
> > I guess it makes more sense to add another patch into the series
> > to do explicit dump of shared_info and then exclude 'special'
> > pages from dump_pageframe_info().
> 
> Bu that's why I said "fragile" - new uses of such pages wouldn't
> automatically be picked up, whereas them all landing on xenpage_list
> made their dumping a reliable thing.
> 

But how useful is dumping xenheap pages this way? There's nothing that actually 
says what they are for so I can't see why it is particularly useful. Having 
something that says 'This is the shared_info page' and 'These are the grant 
shared frames' seems much more desirable... and any new ones added would almost 
certainly merit similar dump functions.

  Paul

> Jan


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