Re: [Xen-devel] [PATCH v2 3/9] xen/mm: move modify_identity_mmio to global file and drop __init

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

On Tue, Apr 25, 2017 at 03:32:50AM -0600, Jan Beulich wrote:
> >>> On 25.04.17 at 11:25, <roger.pau@xxxxxxxxxx> wrote:
> > On Tue, Apr 25, 2017 at 10:09:34AM +0100, Julien Grall wrote:
> >> My understanding is BARs may be allocated by the kernel because the 
> >> firmware
> >> didn't do it. This is the current case on ARM (and I guess x86) where Linux
> >> will always go through the BARs.
> > 
> > No, on x86 BARs are allocated by the firmware. Linux or whatever OS will 
> > scan
> > the BARs in order to get it's position/size, but will not try to move them
> > AFAIK.
> 
> That depends. Firmware is not required to set up all of them (only
> such on devices needed for booting obviously need to be set up).
> And Linux may (voluntarily or forced via command line option) still
> move BARs.

Right. In this series I allow the guest to change the position where the BARs
are mapped into the guest p2m, but I don't allow it to change the physical
address where the BAR is actually mapped. This might work well if all BARs
where positioned by the firmware, but if there are unset BARs I don't think Xen
is capable to make a good decision about it's position (because it lacks
information like ACPI OperationRegions), so I guess I should allow Dom0 to
write directly to the BAR and position it.

BTW, how does Xen deal with MSI-X tables inside of BARs? AFAICT a PV Dom0 is
able to move the BARs around as much as it wants.

Roger.

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