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 16:11, 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.
Anyway, I don't plan to block common code. But I will block any 
implementation of set_foreign_p2m_entry (other than returning not 
implemented) on Arm until someone step up and fix the refcounting.
Cheers,

--
Julien Grall

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