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.

The spec seems more strict here:

"Power-up software needs to build a consistent address map before booting the
machine to an operating system. This means it has to determine how much memory
is in the system, and how much address space the I/O controllers in the system
require. After determining this information, power-up software can map the I/O
controllers into reasonable locations and proceed with system boot."

This is from PCI LOCAL BUS SPECIFICATION, REV. 3.0. I read that as "firmware
will position all the BARs".

Moving the BARs is not a huge problem, this series already allows the guest to
move where the BARs are mapped in it's p2m, allowing a guest to set the initial
BAR position would also be feasible, but I haven't been able to find any device
on my boxes that's not initialized by the firmware, hence it would be hard for
me to test that.

Roger.

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