Re: [Xen-devel] e820_host default value and libxl (not xl)

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, May 23, 2016 at 11:59:10AM +0100, Andrew Cooper wrote:
> On 23/05/16 11:47, Wei Liu wrote:
> > On Sat, May 21, 2016 at 04:42:11AM +0200, Marek Marczykowski-Górecki wrote:
> >> Hi,
> >>
> >> According to xl.cfg(5) " This option defaults to true (1) if any PCI
> >> passthrough devices are configured and false (0) otherwise."
> >> And indeed this behaviour is implemented in xl. But not in libxl, which
> >> means other libxl based toolstacks (libvirt) will not take advantage of
> >> this directly.
> >>
> >> What would be the best approach here? Duplicate that behaviour in
> >> libvirt (currently libvirt knows nothing about this option), or move
> >> that default handling to libxl? I think the later makes more sense, but
> >> maybe there is some reason against it?
> >>
> > The latter.
> >
> > I wouldn't be surprised if the boundary between xl and libxl was
> > overlooked when implementing this flag. I've done similar things to push
> > xsm label handling logic from xl to libxl.
> 
> Please don't propage this bandaid any further than it currently is.  It
> is not appropriate for libxl to set this by default.
> 
> The reason it is currently used is because libxl/libxc doesn't know how
> to lay out a guests physmap.  This is something I am working on
> resolving for some ballooning issues we are having in XenServer.

Does it mean e820_host wouldn't be needed anymore?

> xl can get away defaulting this on, because xl is inherently a single
> host toolstack.  However, using host_e820 is wrong for any multi-host
> setup where the VM might plausibly migrate (which includes the
> passthrough case here).

But *currently* having host_e820 disabled makes it impossible to
passthrough some devices, even when no migration is involved. Libvirt
does not support e820_host. So, I'm looking for a solution for the
problem, which makes it impossible to use some devices at all when using
libvirt.

If e820_host will not be needed anymore in the near future, I can wait,
or simply carry a local patch for this. But otherwise I think it would
be good to fix this in either libxl or libvirt.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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