Re: [PATCH V7 2/2] xen/gnttab: Store frame GFN in struct page_info on Arm

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 11/10/2022 12:59, Jan Beulich wrote:
> On 16.07.2022 16:56, Oleksandr Tyshchenko wrote:
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >
> > Rework Arm implementation to store grant table frame GFN
> > in struct page_info directly instead of keeping it in
> > standalone status/shared arrays. This patch is based on
> > the assumption that a grant table page is a xenheap page.
> >
> > To cover 64-bit/40-bit IPA on Arm64/Arm32 we need the space
> > to hold 52-bit/28-bit + extra bit value respectively. In order
> > to not grow the size of struct page_info borrow the required
> > amount of bits from type_info's count portion which current
> > context won't suffer (currently only 1 bit is used on Arm).
> I'm afraid this isn't true: There's no requirement for a guest to pass
> all different GFNs to VCPUOP_register_vcpu_info, yet map_vcpu_info()
> tries to obtain a reference for every vCPU. 
AFAIU, this would be a reference of the **count_info** not **type_info** 
(which BTW will never be incremented on Arm because we have no type 
support).
The commit message is only referring to the 'type_info's count'. So...

> With my adding of GFN
> (really gaddr) based registration of the runstate area (already
> looking towards 4.18) the maximum possible count is to further grow.
... I am not sure which problem you are referring too.

Cheers,

--
Julien Grall