Re: [Xen-devel] [PATCH 4/5] xen: support different gnttab_max_frames for grant v1 and v2

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.08.17 at 20:05, <jgross@xxxxxxxx> wrote:
> The number of grants a domain can setup is limited by the maximum
> number of grant frames it is allowed to use. Today the limit is the
> same regardless whether the domain uses grant v1 or v2. Using v2
> will therefor be a disadvantage for the domain as only half the
> number of grants compared to v1 can be used, because a grant v2 entry
> is twice as large as the v1 entry. This is the reason for the lack of
> grant v2 support in the Linux kernel (in fact grant v2 support has
> been removed from Linux for this reason).
> 
> OTOH using only grant v1 will limit a pv domain to the low 16TB of
> memory of the host, as grant v1 entries only use a 32 bit mfn. So
> if we want to support more than 16TB of memory and be able to use
> that memory in pv domains, we have to remove the disadvantage of
> using grant v2 by being able to setup the same number of grants as
> with v1.
> 
> In order to achieve this add support for limiting the number of grant
> frames for v1 and v2 independently from each other. Per default let
> the v2 number be twice the value of the v1 number. Modify the boot
> parameter gnttab_max_frames to accept either a single value which
> will set the v1 limit to that value and the v2 limit to 2*value, or
> two values separated by a comma to set both limits to dedicated
> values.
> 
> Add some sanity checks to make sure the maximum number of frames isn't
> lower than the initial number, as this leads to rather strange crashes.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

As discussed elsewhere, this probably rather wants to become a
per-domain setting then. Looking also at what patch 5 adds, I
additionally wonder whether we shouldn't allow Dom0 to know
whether it needs to use v2 at all (or maybe it can derive that
already).

Jan


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