Re: [SeaBIOS] Xen PV block device support in Seabios

[Ian Campbell <ijc@hellion.org.uk>]

(nb, my citrix.com email is no longer valid)
On Wed, 2016-03-16 at 11:33 +0800, Shannon Zhao wrote:
> Hi,
> 
> I noticed there are some efforts to add Xen PV block device support in
> Seabios in a GSoC project and there is a wiki page [1] for it. I found
> some patches [2] to add Xenstore R/W support for Seabios. But I didn't
> find the patches to add PV block device driver in Seabios.
> 
> If you know please tell me where I can find these patches. And I noticed
> that the patches [2] and this GSoC project work didn't get applied to
> Seabios eventually, is there any reason? Does it mean that there are
> some difficulties to support Xen PV block device in Seabios?

This work was never finished, IIRC (and it was a long time ago so this
might be a faulty recollection) the main stumbling block was that there
is no reasonable BIOS level event which could be used to tear down the
PV interfaces in order to hand them over to the OS (unlike, say, EFI
where there is ExitBootServices).

So making this work would require something like a complete set of
parallel PV infrastructure (devices, corresponding xenstore nodes,
grant table) for the use of the BIOS firmware, or perhaps a (tricky to
retrofit in a backwards compatible manner) PV facility for the guest to
reset everything before starting to use them.

Ultimately it didn't seem worth the effort to engineer all that given
that mostly the same aims (faster PV assisted boot) could be achieved
by using UEFI for most modern OSes.

Ian.

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