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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 17 Nov 2016, Julien Grall wrote:
> 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.

Why? Is this a requirement for baremetal guests?

I would have actually opted for always emulating a PL011 even for guests
using a dynamic memory map (which of course one way or another need to
be able to choose the address of the UART, the memory and the rest).


> 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).

Right

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