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



Hi Stefano,

On 05/12/16 20:25, Stefano Stabellini wrote:
On Mon, 5 Dec 2016, Julien Grall wrote:

[...]

But in the case of baremetal application, we want to be able to do logging
only (i.e not reading). They will have a driver for the host UART. It
would be
pointless and potentially difficult to emulate a full UART here. This is
where
vuart come in place (see the use case mentioned by Edgar).

I was suggesting to kill two birds with one stone and just do PL011 for
DomUs (disabled by default, of course). Instead, you would keep the two
use cases separate? PL011 for VM spec guests and vuart for baremetal
apps?

We have to keep those use cases separated. We already went in lengthly
discussion and you agreed on it. Anyway, I will summarize here.

Ops, sorry, after reading the rest of your email I realize that there
was a misunderstanding. Let me explain. I agree that keeping full PL011
emulation and small vuart emulation separate can be a good thing. I also
agree that those are two different use cases.

But I didn't realize that the vuart feature and the expose memory layout
feature were tightly connected for you. I would keep them independent.
In fact, vuart or PL011, I would make a new virtual uart option
available to any guests including the ones without host memory layout,
using an iomem style option. The new uart could be advertised as PL011
on device tree, while in fact only emulating part of it via vuart.c (of
course the fact that doesn't completely emulate a PL011 should be
clearly documented).

On the other hand, you would basically embedded the vuart option into the
host memory layout option and expose it as the same uart as the one on
the host, which of course changes from platform to platform.

Although I like the approach I suggested because I believe it gives
users more flexibility and is less unexpected, because the type of the
uart is well defined, I don't think this is an important decision. The
host memory layout feature is the important one to have and guests that
need a virtual uart will still be able to get one with full PL011
emulation one day. The end results are very similar. So I would leave it
to the contributor or, if you feel strongly about it, I'll leave it up
to you.

After reading what you suggest, I am fine with that. I thought you wanted to only support PL011, hence why I was arguing back.

Cheers,


--
Julien Grall

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