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 George,

On 06/03/17 14:48, George Dunlap wrote:
> On 06/03/17 13:21, Julien Grall wrote:
> > 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?
> Right, but in order to evaluate your question about whether we're
> "happy" with not being able to use these parameters if we ever decide to
> emulate UART on x86, we need to know a reason that one might decide to
> add a UART *on x86*, not on ARM.
>
> I mean, I don't think we're running out of bits, so I don't think it's a
> problem allocating new HVM_PARAM numbers so that we can re-use them if
> we ever decide to implement UARTs on x86.  On the other hand, given that
> nobody has ever suggested doing so or knows why it might be useful,
> maybe it makes more sense to just re-use some x86 params and allocate
> extra numbers if / when we decide we want to implement UART on x86.
I agree that the reason I gave is very ARM focused. I don't know what is 
the future on Xen for x86, but on ARM we have another potential use case 
for the UART emulation which I think could also apply for x86.
On ARM, the guest is booting through the same path as on native. All the 
devices are discovered through the firmware tables. So it would be 
possible to run a guest OS without any knowledge of Xen if you assign 
all the necessary physical devices (e.g disk, network, serial...). Often 
you want to log the console of all your guests and keep them safely in 
the controller domain. This is where an emulated UART can be useful, you 
don't need to add Xen drivers in the guest and still can use the guest 
seamlessly via xl.
Cheers,

--
Julien Grall

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