Re: [Xen-devel] [PATCH v12 05/11] x86/mm: add HYPERVISOR_memory_op to acquire guest resources

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

On 19/10/17 16:47, Jan Beulich wrote:
> > > > On 19.10.17 at 17:37, <julien.grall@xxxxxxxxxx> wrote:
> > Hi,
> >
> > On 19/10/17 16:11, Jan Beulich wrote:
> > > > > > On 19.10.17 at 16:49, <Paul.Durrant@xxxxxxxxxx> wrote:
> > > > > > I'd prefer to make the whole thing x86-only since that's the only platform
> > > > > on which I can test it, and indeed the code used to be x86-only. Jan
> > objected
> > > > > to this so all I'm trying to achieve is that it builds for ARM. Please can
> > you and
> > > > > Jan reach agreement on where the code should live and how, if at all, it
> > > > > should be #ifdef-ed?
> > > > >
> > > > > I am quite surprised of "it is tools-only" so it is fine to not protect
> > > > > it even if it is x86 only. That's probably going to bite us in the future.
> > > > So, this appears to have reached an impasse. I don't know how to proceed
> > > > without having to also fix priv mapping for x86, which is a yak far too
> > large
> > > > for me to shave at the moment.
> > > Julien,
> > >
> > > why is it that you are making refcounting on p2m insertion / removal
> > > a requirement for this series? We all know it's not there right now
> > > (and similarly also not for the IOMMU, which might affect ARM as well
> > > unless you always use shared page tables), and it's - as Paul validly
> > > says - unrelated to his series.
> > Well, we do at least have refcounting for foreign mapping on Arm. So if
> > a foreign domain remove the mapping, the page will stay allocated until
> > the last mapping is removed. For IOMMU, page tables are for the moment
> > always shared.
> >
> > If you don't want to fix x86 now, then it is fine. But I would
> > appreciate if you don't spread that on Arm.
> >
> > To give you a bit more context, I was ready to implement the Arm version
> > of set_foreign_p2m_entry (it is quite trivial) to append at the end of
> > this series. But given that refcounting is not done, I am more reluctant
> > to do that.
> I don't understand: The refcounting is to be done by ARM-specific
> code anyway, i.e. by the implementation of set_foreign_p2m_entry(),
> not its caller. At least that's what I would have expected.
I thought I said it before, but it looks like not. Assuming the MFN is 
always baked by a domain, the prototype would likely need to be extended 
and take the foreign domain.
If it is not the case, we would need to find another way to do refcounting.

Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel