Re: [Xen-devel] [PATCH 5/5] xen: add new hypercall to get grant table limits

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.08.17 at 20:05, <jgross@xxxxxxxx> wrote:
> Today a guest can get the maximum grant table frame number for the
> currently selected grant table interface version (1 or 2) only. Add
> a new grant table operation to get the limits of both variants in
> order to give the guest an opportunity to select the version serving
> its needs best.
> 
> Background for the need for this new hypercall is that e.g. the Linux
> kernel won't use v2 as long as this will allow less active grants as
> v1. As soon as the kernel can detect it can create as many v2 entries
> as for v1, it will have no reason not to use v2. And this will allow
> Xen placing a pv-domain with such a kernel above the current 16TB
> RAM limit.
> 
> For setting up the grant table a kernel needs the following
> information:
> - current version (kexec case)
> - current size (kexec case)
> - max size v1
> - max size v2
> In order not to have to issue 3 hypercalls (GNTTABOP_query_size,
> GNTTABOP_get_version, GNTTABOP_get_v1_and_v2_max), let the new
> hypercall return all the needed information.

I'm not sure I follow: v2 is always going to allow less active grants
than v1, as the limit is always derived from the number of frames
allowed for a domain. I also don't see a problem with issuing multiple
calls - none of this ought to be performance critical. I would,
however, agree that rather than adding a new hypercall to just
return the max sizes adding one like you suggest would be
preferable. I'm simply not convinced yet that returning the max
sizes is actually necessary.

Jan


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