Re: [PATCH V4 05/24] xen/ioreq: Make x86's hvm_ioreq_needs_completion() common

[Julien Grall <julien@xxxxxxx>]

Hi Alex,

On 20/01/2021 08:48, Alex Bennée wrote:
> Oleksandr Tyshchenko <olekstysh@xxxxxxxxx> writes:
>
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >
> > The IOREQ is a common feature now and this helper will be used
> > on Arm as is. Move it to xen/ioreq.h and remove "hvm" prefix.
> >
> > Although PIO handling on Arm is not introduced with the current series
> > (it will be implemented when we add support for vPCI), technically
> > the PIOs exist on Arm (however they are accessed the same way as MMIO)
> > and it would be better not to diverge now.
> I find this description a little confusing. When you say PIO do you mean
> using instructions like in/out on the x86? If so then AFAIK it's a
> legacy feature of x86 as everything I've come across since just does
> MMIO, including PCI.
Stefano and I had quite a long discussion about this a few months ago 
(see [1]).
From my understanding, while Arm will access the PCI I/O BAR via MMIO, 
the BAR itself will be configured using an offset from a fixed I/O 
window base address. IOW, we don't configure the BAR with a full MMIO 
address.
In the case the hostbridge is emulated in Xen, I would like to re-use 
the TYPE_PIO for such access because it makes the device model 
arch-agnostic.
I believe this would behave the same way as a real PCI device card: you 
can plug it anywhere without having to understand the underlying 
architecture.
If we were going to use the MMIO type, then we would need:
  1) Inform each device model where is the I/O Window (necessary to be 
able to know we are accessing the I/O BAR)
  2) Have arch boiler plate in the device model

> The code changes look fine to me though:
>
> Reviewed-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
Cheers,

[1] <4cbe37bd-abd2-3d02-535e-cca6f7497ef2@xxxxxxx>


--
Julien Grall