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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.