Re: [PATCH] xen/arm: fix SBDF calculation for vPCI MMIO handlers

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Oct 27, 2021 at 09:04:39AM +0000, Oleksandr Andrushchenko wrote:
> Hi, Roger!
> 
> On 27.10.21 11:59, Roger Pau Monné wrote:
> > On Wed, Oct 27, 2021 at 11:25:33AM +0300, Oleksandr Andrushchenko wrote:
> >> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> >>
> >> While in vPCI MMIO trap handlers for the guest PCI host bridge it is not
> >> enough for SBDF translation to simply call VPCI_ECAM_BDF(info->gpa) as
> >> the base address may not be aligned in the way that the translation
> >> always work.
> > I think I've already raised this during review [0]. But this is only a
> > problem if you change the current value of GUEST_VPCI_ECAM_BASE
> > AFAICT, as the current value has bits [0,27] clear.
> Exactly, so we were just lucky not to hit this before
> >
> > I assume this is a problem for the hardware domain that needs to trap
> > random base addresses as present on hardware, but that code hasn't
> > been committed yet.
> Yes, I am facing this on the real HW when Dom0's access is trapped
> and the base is not taken into account. So, I have a patch for the
> future upstream which subtracts the relevant base from the gpa,
> e.g. either GUEST_VPCI_ECAM_BASE or bridge->cfg->phys_addr
> >
> > If that's indeed the case, please expand the commit message to contain
> > this information.
> I can only mention about "the current value of GUEST_VPCI_ECAM_BASE
> AFAICT, as the current value has bits [0,27] clear" as of now because
> Dom0 traps are not yet there.

Indeed, I would rather mention both cases, ie:

"This change is not strictly required given the current value of
GUEST_VPCI_ECAM_BASE which has bits 0 to 27 clear, but could cause
issues if such value is changed, or when handlers for dom0 ECAM
regions are added as those will be mapped over existing hardware
regions that could use non-aligned base addresses."

Regards, Roger.