Re: [PATCH v2 1/7] xen/mm: lift 32 item limit from mfn/gfn arrays

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.06.2020 13:36, Michał Leszczyński wrote:
> ----- 19 cze 2020 o 13:34, Roger Pau Monné roger.pau@xxxxxxxxxx napisał(a):
> 
>> On Fri, Jun 19, 2020 at 01:38:00AM +0200, Michał Leszczyński wrote:
>>> Replace on-stack array allocation with heap allocation
>>> in order to lift the limit of 32 items in mfn/gfn arrays
>>> when calling acquire_resource.
>>
>> I'm afraid this is not correct, you cannot allow unbounded amounts of
>> items to be processed like this, it's likely that you manage to
>> trigger the watchdog if the list is long enough, specially when doing
>> set_foreign_p2m_entry.
>>
>> You need to process the items in batches (32 was IMO a good start), and
>> then add support for hypercall continuations. Take a look at how
>> XENMEM_populate_physmap just a couple of lines below makes use of
>> hypercall_create_continuation.
>>
>> After processing every batch you need to check if
>> hypercall_preempt_check returns true and if so use
>> hypercall_create_continuation in order to encode a continuation.
>>
>> Thanks, Roger.
> 
> 
> Somebody previously suggested that this limit could be lifted this way,
> so I would like to hear some more opinions on that.

I did suggest the limit can be lifted, but not by processing all
pieces in one go. Whether batches of 32 or 64 or 128 are chosen
is a different thing, but you can't do arbitrary amounts without
any preemption checks.

Jan