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 14:28, Jan Beulich wrote:
> On 11.10.2022 15:01, Julien Grall wrote:
> > 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).
> I should have said "obtain a writable type reference".
Thanks for the clarification.

> > 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.
> Wow - a mere stub (but not inline) function to make the build happy.
> Then why is the description talking about one bit that's needed on
> Arm?
Because share_xen_page_with_guest() will always set the type info's 
count to 1.
TBH I don't exactly know why we set it. I always assumed this was a 
requirement for the common code but never checked.
Cheers,


--
Julien Grall