Re: [QEMU][PATCH v2 10/11] hw/arm: introduce xenpv machine

[Julien Grall]

Hi,

On 02/12/2022 22:36, Stefano Stabellini wrote:
> > Do you know what Xen version your build env has?
>
> I think Alex is just building against upstream Xen. GUEST_TPM_BASE is
> not defined there yet. I think we would need to introduce in
> xen_common.h something like:
>
> #ifndef GUEST_TPM_BASE
> #define GUEST_TPM_BASE 0x0c000000
> #endif

I think this would be a big mistake to add the two lines above in QEMU.

Libxl is responsible for creating the domain and generating the firwmare 
tables. Any mismatch of values will be a real pain to debug.

Even if...

> We already have similar code in xen_common.h for other things.  Also, it
> would be best to get GUEST_TPM_BASE defined upstream in Xen first.

... we introduce upstream first, the guest layout is not part of the 
stable ABI and therefore could change from release to release.

> > Another way to fix this(as Julien suggested) is by setting this GUEST_TPM_BASE
> > value via a property or something and user can set it via command line.
> >
> > @sstabellini@kernel.org, do you think of any other fix?
>
> Setting the TPM address from the command line is nice and preferable to
> hardcoding the value in xen_common.h. It comes with the challenge that
> it is not very scalable (imagine we have a dozen emulated devices) but
> for now it is fine and a good way to start if you can arrange it.

It is not clear which one you think is not scalable. If this is the 
command line option approach, then I think this is unrealistic to ask 
every user to rebuild there QEMU just because the guest layout has changed.

Today the rebuild may only be necessary when switching to a new release. 
But in the future we may imagine a per-domain layout (e.g. for legacy 
purpose). So you will now need to request the user to have one QEMU 
built per domain.

How is that scalable?

Cheers,

--
Julien Grall