Re: [Xen-devel] [PATCH 02/11] xen/arm: vpl011: Add new hvm params in Xen for ring buffer/event setup

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 06/03/17 12:41, Jan Beulich wrote:
> > > > On 06.03.17 at 12:42, <julien.grall@xxxxxxx> wrote:
> > I thought a bit more about those params. I think the name should be
> > generic and not tie to pl011 because we may want to emulate different
> > UART for the guest in the future.
> That's reasonable, but I continue to have reservations against the
> underlying approach here, namely ...
>
> > Also, by re-using deprecated encoding it means that it will not be
> > possible to use those parameters on x86 if you ever decide to emulate
> > UART in Xen. I am not sure whether if you are happy with that?
> ... with this in mind: If we wanted to do this for HVM guests, we'd
> indeed want the parameters. If we wanted this for PV guests, the
> model wouldn't fit at all.
>
> Plus what I'm not understanding (perhaps because most of the
> discussion around this seemed to be ARM-specific, and hence I've
> skipped reading through it) is - why is this UART emulation needed
> in the first place? We've never had a need for such on x86, afaia.
Linaro has published a set of guidelines to guarantee a same guest image 
can run on multiple hypervisors (see [1]). This specification mandates 
the presence of a SBSA-compliant UART.
I just realized the cover letter does not explain why we need to emulate 
a PL011 on ARM. Bhupinder can you detail it on the next version?
> Furthermore - how does this scale? I.e. what if other devices pop
> up wanting to be emulated? Or multiple UARTs?
I think you can appreciate that using QEMU for emulating an UART is 
quite overkill.
Also, we are expecting more people to look at providing mediation (such 
as Firmware access) within Xen as they are concerned about performance 
and certification.
An idea to ensure the integrity of Xen would be to run this code at a 
lower level.
Cheers,

[1] 
https://www.linaro.org/app/resources/WhitePaper/VMSystemSpecificationForARM-v2.0.pdf
--
Julien Grall

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