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Re: [PATCH v2] gnttab: defer allocation of status frame tracking array

Hi Andrew and Jan,

On 04/01/2021 15:41, Andrew Cooper wrote:
On 04/01/2021 15:22, Jan Beulich wrote:
On 04.01.2021 16:04, Andrew Cooper wrote:
On 23/12/2020 15:13, Jan Beulich wrote:
This array can be large when many grant frames are permitted; avoid
allocating it when it's not going to be used anyway, by doing this only
in gnttab_populate_status_frames().

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
v2: Defer allocation to when a domain actually switches to the v2 grant
I see this as a backwards step.  It turns a build-time -ENOMEM into a
runtime -ENOMEM, and if you recall from one of the XSAs, the Windows PV
drivers will BUG() if set_version fails.  (Yes - this is dumb behaviour,
but it is in the field now.)

During yesterday call, Paul pointed that this is a behavior from old Windows driver. New Windows PV driver will not use Grant Table v2.

However, AFAICT, there is already runtime -ENOMEM failure in set_version when trying to populate the status frame (see the call to gnttab_populate_status_frames()).

Well, if this was the only source of -ENOMEM (i.e. none was there
previously), I'd surely understand the concern. But there have been
memory allocations before on this path.

... you're literally writing a patch saying "avoid large allocation at
domain create time, and make it at runtime instead" and then trying to
argue that it isn't a concern because there are other memory allocations.

It is very definitely a backwards step irrespective of the size of the
allocation, even if the current behaviour isn't necessarily perfect.

  In any event, this will
need settling between you and Julien, as it was him to request the

Well - that's because gnttab v2 is disabled in general on ARM.

I didn't have Arm in mind when I originally requested Jan the change.

Instead, the request was based on the fact that most of the guests don't use of grant-table v2. To me this feels a waste of memory and if it can be avoid then it is best.

Conditionally avoiding the allocation because of opt_gnttab_max_version
being 1 would be ok, because it doesn't introduce new runtime failures
for guests.

Based on my answer above, I would not consider it as a new runtime failure -ENOMEM is already possible when switching from v1 to v2.

In fact, the allocation is going to be much smaller than the other allocations (we may be allocating multiple pages). So if we are concerned about this, then we should already be concerned about the existings one.

Anyway, the array itself is likely going to be small (I haven't computed the size) so I would be OK to not defer the allocation.

However, I would like to seek clarification on whether your end goal is to remove any -ENOMEM failure from set_version.


Julien Grall



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