Re: [PATCH v6 01/13] xen/pci: arm: add stub for is_memory_hole

[Julien Grall <julien@xxxxxxx>]

On 04/02/2022 09:01, Oleksandr Andrushchenko wrote:
> On 04.02.22 10:51, Julien Grall wrote:
> > Hi,
> >
> > On 04/02/2022 06:34, Oleksandr Andrushchenko wrote:
> > > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> > >
> > > Add a stub for is_memory_hole which is required for PCI passthrough
> > > on Arm.
> > >
> > > Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> > >
> > > ---
> > > Cc: Julien Grall <julien@xxxxxxx>
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > ---
> > > New in v6
> > > ---
> > >    xen/arch/arm/mm.c | 6 ++++++
> > >    1 file changed, 6 insertions(+)
> > >
> > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > > index b1eae767c27c..c32e34a182a2 100644
> > > --- a/xen/arch/arm/mm.c
> > > +++ b/xen/arch/arm/mm.c
> > > @@ -1640,6 +1640,12 @@ unsigned long get_upper_mfn_bound(void)
> > >        return max_page - 1;
> > >    }
> > >    +bool is_memory_hole(mfn_t start, mfn_t end)
> > > +{
> > > +    /* TODO: this needs to be properly implemented. */
> > I was hoping to see a summary of the discussion from IRC somewhere in the patch 
> > (maybe after ---). This would help to bring up to speed the others that were 
> > not on IRC.
> I am not quite sure what needs to be put here as the summary
At least some details on why this is a TODO. Is it because you are 
unsure of the implementation? Is it because you wanted to send early?...
IOW, what are you expecting from the reviewers?

> Could you please help me with the exact message you would like to see?
Here a summary of the discussion (+ some my follow-up thoughts):

is_memory_hole() was recently introduced on x86 (see commit 75cc460a1b8c 
"xen/pci: detect when BARs are not suitably positioned") to check 
whether the BAR are positioned outside of a valid memory range. This was 
introduced to work-around quirky firmware.
In theory, this could also happen on Arm. In practice, this may not 
happen but it sounds better to sanity check that the BAR contains 
"valid" I/O range.
On x86, this is implemented by checking the region is not described is 
in the e820. IIUC, on Arm, the BARs have to be positioned in pre-defined 
ranges. So I think it would be possible to implement is_memory_hole() by 
going through the list of hostbridges and check the ranges.
But first, I'd like to confirm my understanding with Rahul, and others.

If we were going to go this route, I would also rename the function to 
be better match what it is doing (i.e. it checks the BAR is correctly 
placed). As a potentially optimization/hardening for Arm, we could pass 
the hostbridge so we don't have to walk all of them.
Cheers,

--
Julien Grall