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

[Juergen Gross <jgross@xxxxxxxx>]

On 24/08/17 16:48, Juergen Gross wrote:
> On 24/08/17 16:28, Jan Beulich wrote:
>>>>> 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.
> 
> Right. Patch 3 adds support for different number of allowed frames for

This should read "Patch 4", of course

> v1 and v2. So the system can be configured to allow the same max.
> number of grants for v1 and v2, or even more v2 grants than v1.
> 
>> 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.
> 
> How would the guest know whether using v2 grants is no disadvantage?
> 
> 
> Juergen
> 
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> 


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