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

[Juergen Gross <jgross@xxxxxxxx>]

On 24/08/17 17:20, Jan Beulich wrote:
>>>> On 24.08.17 at 17:13, <jgross@xxxxxxxx> wrote:
>> On 24/08/17 17:04, Jan Beulich wrote:
>>>>>> On 24.08.17 at 16:48, <jgross@xxxxxxxx> wrote:
>>>> How would the guest know whether using v2 grants is no disadvantage?
>>>
>>> As said - it's always going to be a disadvantage. Even if controlling
>>> the number of frames per-domain, that same number of frames
>>> will always fit more v1 than v2 entries.
>>
>> And my patches break the assumption "same number of frames".
>>
>>> I don't think the config
>>> setting should directly control the number of active grants.
>>
>> Why not? In the end that number is the one the guest is interested in.
> 
> And the resource use is what the admin is interested in.
> 
>> BTW: the number of needed maptrack frames is depending on the number
>> of grants only, not on v1 or v2. And the default for the max. number
>> of maptrack frames is much higher than the one of the grant frames
>> (1024 vs 32).
> 
> Of course.

So what about the following idea:

- Set the default max number of grant frames to 32 on "small" hosts
  (max. physical memory address below 16TB) and to 64 on "large"
  hosts
- Add a per-domain parameter for the max number of grant frames

In the Linux kernel I would then:

- Re-add grant v2 support
- Add boot parameter for selecting grant v1 or v2
- Use grant v2 if selected via boot parameter, or (pv only:) max. host
  address is above 16TB


Juergen

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