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

[Juergen Gross <jgross@xxxxxxxx>]

On 24/08/17 16:21, Jan Beulich wrote:
>>>> 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).

I'm absolutely in favor of adding a way to make this a per-domain
setting.

OTOH I think there should be a default especially on huge hosts
allowing to use v2 grants without reducing the number of allowed
grants, which doesn't need adding another parameter to the domain
config. Or do you want the tools to always set the per-domain value
possibly based on a xl.conf value? Then we could remove the
gnttab_max_frames command line parameter completely.


Juergen

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