Re: [Xen-devel] Upstream Dom0 DRM problems regarding swiotlb

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.02.19 at 18:45, <michael.d.labriola@xxxxxxxxx> wrote:
> On Wed, Feb 13, 2019 at 11:09 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>> >>> On 13.02.19 at 17:00, <michael.d.labriola@xxxxxxxxx> wrote:
>> > On Wed, Feb 13, 2019 at 9:28 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> >> >>> On 13.02.19 at 15:10, <michael.d.labriola@xxxxxxxxx> wrote:
>> >> > Ah, so this isn't necessarily Xen-specific but rather any paravirtual
>> >> > guest?  That hadn't crossed my mind.  Is there an easy way to find out
>> >> > if we're a pv guest in the need_swiotlb conditionals?
>> >>
>> >> There's xen_pv_domain(), but I think xen_swiotlb would be more to
>> >> the point if the check is already to be Xen-specific. There's no generic
>> >> "is PV" predicate that I'm aware of.
>> >
>> > Well, that makes doing conditional code right more difficult.  I
>> > assume since there isn't a generic predicate, and PV isn't new, that
>> > it's absence is by design?  To reign in the temptation to sprinkle
>> > conditional code all over the kernel?  ;-)
>>
>> Well, with lguest gone, Xen is the only PV environment the kernel
>> can run in, afaik at least. I guess to decide between the suggested
>> options or the need for some abstracting macro (or yet something
>> else), you'll really need to ask the driver maintainers. Or simply
>> send a patch their way implementing one of them, and see what
>> their reaction is.
> 
> Thanks, I'll do that.
> 
> When you said any PV guest would need swiotlb, not just Xen, does that
> mean anything that's using CONFIG_PARAVIRT?  That appears to include
> KVM, VMware, Xen PVH, and Xen HVM in addition to Xen PV, all of which
> populate the global pv_info structure at kernel bootup.  Is Xen PV the
> only one of those that requires swiotlb?

No - paravirtual interfaces can also be used by fully virtualized guests.
As said, to my knowledge Xen is the only environment in which the
Linux kernel supports running paravirtualized.

Jan



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