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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper
> Sent: 14 March 2019 19:37
> To: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>; Jason Andryuk 
> <jandryuk@xxxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>
> Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; qemu-devel@xxxxxxxxxx; 
> marmarek@xxxxxxxxxxxxxxxxxxxxxx
> 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 <Paul.Durrant@xxxxxxxxxx> 
> >> wrote:
> >>>> -----Original Message-----
> >>>> From: Jason Andryuk [mailto:jandryuk@xxxxxxxxx]
> >>>> Sent: 11 March 2019 18:02
> >>>> To: qemu-devel@xxxxxxxxxx
> >>>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; marmarek@xxxxxxxxxxxxxxxxxxxxxx; 
> >>>> Simon Gaiser
> >>>> <simon@xxxxxxxxxxxxxxxxxxxxxx>; Jason Andryuk <jandryuk@xxxxxxxxx>; 
> >>>> Stefano Stabellini
> >>>> <sstabellini@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; 
> >>>> Paul Durrant
> >>>> <Paul.Durrant@xxxxxxxxxx>
> >>>> Subject: [PATCH 6/6] xen-pt: Round pci regions sizes to XEN_PAGE_SIZE
> >>>>
> >>>> From: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
> >>>>
> >>>> 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
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