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

[Ian Campbell <ijc@xxxxxxxxxxxxxx>]

On Wed, 2016-03-16 at 20:13 +0800, Shannon Zhao wrote:
> 
> On 2016/3/16 19:20, Ian Campbell wrote:
> > 
> > (nb, my citrix.com email is no longer valid)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).
> > 
> Ian, thanks for your reply! It looks like the problem is how and when to
> clear PV resources in seabios before handing over to guest. But I wonder
> why virtio works in seabios. Does seabios using virtio need to clear
> things like vrings? Or seabios doesn't clear the things and guest just
> covers the configuration with new values?

I think virtio covered this use case from day 1 by having the reset,
but also by not having a xenstore ring etc.

> > 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.
> > 
> Like guest front-end driver checks if the backend state is
> XenbusStateInitWait, if not, tell the backend to reset to
> XenbusStateInitWait state?

Before it can do this the guest needs to recover the xenbus ring (which
was used by SeaBIOS) into a usable state -- so you need to be thinking
at least one layer further down before you can start to think about
individual devices, and don't forget the grant table (which may have in
use entries from the BIOS) and event channels (which the BIOS may have
setup).

I'm afraid I don't have any concrete answer for what exactly needs to
be done to make this work, but I do know that it is a (IMHO very) non-
trivial amount of investigation, prototyping and coding.

> > 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.
> Yes, for modern OSes it could UEFI. But for some old OSes or some
> existing VMs, if we want to use PV assisted boot, it needs to add this
> in seabios.

Sure, it's a completely reasonable feature to want, but you should be
aware that there are some very non-trivial issue to be resolved, at
which point you need to ask whether it is worth your time and effort.

Ian.

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