Re: [PATCH v2 1/3] xen/arm: pci: introduce PCI_PASSTHROUGH Kconfig option

[Julien Grall <julien@xxxxxxx>]

Hi,

On 18/07/2023 18:35, Stewart Hildebrand wrote:
> On 7/13/23 14:40, Julien Grall wrote:
> > Hi Stewart,
> >
> > On 07/07/2023 02:47, Stewart Hildebrand wrote:
> > > From: Rahul Singh <rahul.singh@xxxxxxx>
> > >
> > > Setting CONFIG_PCI_PASSTHROUGH=y will enable PCI passthrough on ARM, even though
> > > the feature is not yet complete in the current upstream codebase. The purpose of
> > > this is to make it easier to enable the necessary configs (HAS_PCI, HAS_VPCI) 
> > > for
> > > testing and development of PCI passthrough on ARM.
> > >
> > > Since PCI passthrough on ARM is still work in progress at this time, make it
> > > depend on EXPERT.
> > While preparing the patch for committing, I noticed that HAS_PASSTHROUGH
> > will now allow the user to select one of the IOMMU quarantine options.
> >
> > There are three of them right now:
> >    1. none
> >    2. basic (i.e. faulting)
> >    3. scratch page
> >
> > The latter is unlikely to work on Arm because we don't setup the scratch
> > page. AFAIU, for that, we would need to implement the callback
> > quarantine_init().
> >
> > I would expect 1 and 2 work. That said, I think 1. would behave like 2.
> > because on Arm the device should not be automatically re-assigned to
> > dom0. I know this is correct for platform device, but will it be valid
> > for PCI as well?
> In a system with dom0 where the guest is created from the xl toolstack, we rely on 
> "xl pci-assignable-add". Upon domain destruction, the device automatically gets 
> assigned to domIO.
Ok. To clarify, does this mean any DMA will fault, the same as for 
platform device?
> However, there's nothing preventing a user from attempting to invoke "xl 
> pci-assignable-remove", which should assign the device back to dom0, but it is not 
> automatic.
I don't think we want to fully prevent a user to re-assign a device to 
dom0. But we at least want to avoid re-assigning the device to dom0 by 
default. After that a user can reset the device before it gets 
re-assigned to dom0.
Cheers,

--
Julien Grall