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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, May 23, 2016 at 01:27:31PM +0200, Marek Marczykowski-Górecki wrote:
> 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?

Presumarily also the other flags we have: mmio_hole ?
> 
> > 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).

It would be better if you could give the libxl/xl a e820 array record.

As in - one could jam an e820 that would be the same across various
servers.

> 
> 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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