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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.03.16 at 14:15, <konrad.wilk@xxxxxxxxxx> wrote:
> On Wed, Mar 16, 2016 at 12:22:32PM +0000, Ian Campbell wrote:
>> 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 (nb, my citrix.com email is no longer valid)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.
> 
> But it does work with OVMF. That is it implements the full PV driver
> for block. So it surely is possible.

And grub2, iirc.

Jan


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