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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

Jan

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