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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

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).

~Andrew

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