Re: [Xen-devel] Xen ARM small task (WAS: Re: [Xen Question])

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 17/11/2016 11:26, Stefano Stabellini wrote:
> On Mon, 14 Nov 2016, Julien Grall wrote:
> > On 11/11/2016 13:55, Stefano Stabellini wrote:
> > > On Fri, 11 Nov 2016, Julien Grall wrote:
> > > > On 11/11/2016 02:24, Stefano Stabellini wrote:
> > > > > On Thu, 10 Nov 2016, Julien Grall wrote:
> > > > > > (CC Stefano and change the title)
> > > > > > On 10/11/16 12:13, casionwoo wrote:
> > > > > > > I’m pleased about your reply and you have a lot of code to clean-up.
> > > > > > >
> > > > > > > As you mentioned, It’s really huge to digest at once. Thank you for
> > > > > > > understanding me.
> > > > > > >
> > > > > > > And that’s why i need a small fix up and todo list.
> > > > > > >
> > > > > > > I feel familiar with ARM and c language and there’s no specific area
> > > > > > > yet.
> > > > > > >
> > > > > > > I think that i can find interesting area with following up the
> > > > > > > codes.
> > > > > > >
> > > > > > > I’m looking forward to being stuck on Xen.
> > > > > > >
> > > > > > > Then it would be easier for me to understand about Xen on ARM.
> > > > > > >
> > > > > > > Please let me know the TODO and bug-fix lists.
> > > > > > Stefano, before giving a list of code clean-up, do you have any small
> > > > > > TODO
> > > > > > on
> > > > > > ARM in mind?
> > > > > A simple task we talked about recently is to enable the vuart
> > > > > (xen/arch/arm/vuart.c) for all guests. At the moment it is only emulated
> > > > > for Dom0, but some guests, in particular BareMetal guests
> > > > > (https://en.wikipedia.org/wiki/BareMetal), would benefit from it.
> > > > >
> > > > > It would be best to introduce an option in libxl to explicitly
> > > > > enable/disable the vuart for DomUs. Something like vuart=1 in the VM
> > > > > config file.
> > > > The vuart has not been enabled for DomU because it the UART region may
> > > > clash
> > > > with the guest memory layout (which is static).
> > > >
> > > > I don't think this option should be available until we allow the guest to
> > > > use
> > > > the same memory layout as the host (see e820_host parameter for x86).
> > > Actually there is no reason for the vuart to use the same address as
> > > the physical uart on the platform, is there?
> > > In fact it doesn't even
> > > have to prentend to be the same uart as the one on the board, right?
> > > The vuart MMIO address could be completely configurable and independent
> > > from the one of the physical uart.
> > There is no reason to use the same information as the physical UART.
> >
> > However, the vuart requires quite a few information (e.g base address, offset
> > of different register... see vuart_info structure in include/xen/serial.h for
> > more details) in order to fully work.
> >
> > IHMO this is a lot of works for the user to configure. I would much prefer to
> > see a PL011 emulated at a specific base address and let the user select
> > whether he wants a UART to debug or not.
> So you envision the configuration of the MMIO base address to be done as
> part of a new dynamic guest memory map?
For guest using dynamic memory map, I would expect to expose an 
uncompleted emulation of the physical UART (e.g it would only be 
possible to write) at the exact same address as on the host.
For guest using static memory map (i.e the current approach), I would 
envision to always emulate a PL011 (or at least giving this option) and 
not the physical UART.
This has a better fit with the support of SBSA/VM Spec compliant guest 
and still allow a user to do early debugging. Also, by always exposing 
the same kind of UART, the user does not have to know what is the 
underlying platform (e.g which UART is used).
Regards,

--
Julien Grall

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