Re: [Qemu-devel] [PATCH 6/6] xen-pt: Round pci regions sizes to XEN_PAGE_SIZE

[Paul Durrant]

> -----Original Message-----
> From: Andrew Cooper
> Sent: 14 March 2019 19:37
> To: Simon Gaiser <address@hidden>; Jason Andryuk <address@hidden>; Paul 
> Durrant
> <address@hidden>
> Cc: Anthony Perard <address@hidden>; address@hidden; Stefano Stabellini
> <address@hidden>; address@hidden; address@hidden
> Subject: Re: [PATCH 6/6] xen-pt: Round pci regions sizes to XEN_PAGE_SIZE
> 
> On 14/03/2019 19:22, Simon Gaiser wrote:
> > Jason Andryuk:
> >> On Wed, Mar 13, 2019 at 11:09 AM Paul Durrant <address@hidden> wrote:
> >>>> -----Original Message-----
> >>>> From: Jason Andryuk [mailto:address@hidden
> >>>> Sent: 11 March 2019 18:02
> >>>> To: address@hidden
> >>>> Cc: address@hidden; address@hidden; Simon Gaiser
> >>>> <address@hidden>; Jason Andryuk <address@hidden>; Stefano Stabellini
> >>>> <address@hidden>; Anthony Perard <address@hidden>; Paul Durrant
> >>>> <address@hidden>
> >>>> Subject: [PATCH 6/6] xen-pt: Round pci regions sizes to XEN_PAGE_SIZE
> >>>>
> >>>> From: Simon Gaiser <address@hidden>
> >>>>
> >>>> If a pci memory region has a size < XEN_PAGE_SIZE it can get located at
> >>>> an address which is not page aligned.
> >>> IIRC the PCI spec says that the minimum memory region size should be at 
> >>> least 4k. Should we even
> be tolerating BARs smaller than that?
> >>>
> >>>   Paul
> >>>
> >> Hi, Paul.
> >>
> >> Simon found this, so it affects a real device.  Simon, do you recall
> >> which device was affected?
> > Not sure which one it was. Probably the USB controller or the SD host
> > controller. As your example below shows this is not so uncommon.
> 
> The minimum is 128 bytes, not 4k - I've just checked the PCIe spec.
> 
> Xen/Qemu definitely needs to cope with smaller than 4k if we want to be
> spec compliant.

Well, we have a problem for pass-through if the BAR is smaller than 4k in that 
page protection is not going to isolate it. I don't see any other way that to 
trap and emulate such BARs if we want to pass through those devices at all.

  Paul

> 
> ~Andrew