Re: [Xen-devel] IOREQ server on Arm

[Julien Grall <julien.grall@xxxxxxx>]

Hi Paul,

On 09/26/2018 11:51 AM, Paul Durrant wrote:
> > -----Original Message-----
> > From: Julien Grall [mailto:julien.grall@xxxxxxx]
> > Sent: 26 September 2018 11:41
> > To: Jan Beulich <JBeulich@xxxxxxxx>; Paul Durrant
> > <Paul.Durrant@xxxxxxxxxx>
> > Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Roger Pau Monne
> > <roger.pau@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; xen-
> > devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> > Subject: Re: IOREQ server on Arm
> >
> > Hi Jan,
> >
> > On 09/26/2018 09:08 AM, Jan Beulich wrote:_
> > > > > > On 26.09.18 at 00:39, <julien.grall@xxxxxxx> wrote:
> > > > Hi Paul,
> > > >
> > > > I am looking at porting the IOREQ server infrastructure on Arm. I
> > didn't
> > > > need much modification to make it run for Arm. Although, the
> > > > implementation could be simplified over the x86 implementation.
> > > >
> > > > I noticed some issue while trying to implement the hypercall
> > > > XENMEM_acquire_resource. Per my understanding, all the page mapped via
> > > > that hypercall will use the type p2m_mapping_foreign.
> > > >
> > > > This will result to trigger the ASSERT(fdom != dom) in
> > get_page_from_gfn
> > > > (asm-arm/p2m.h) because the IOREQ page has been allocated to the
> > > > emulator domain and mapped to it. AFAICT x86 has the same assert in
> > > > p2m_get_page_from_gfn(...).
> > > >
> > > > IHMO, the ASSERT makes sense because you are only meant to map page
> > > > belonging to other domain with that type.
> > > >
> > > > So I am wondering whether IOREQ server running in PVH Dom0 has been
> > > > tested? What would be the best course of action to fix the issue?
> > > I think the p2m type needs to be chosen based on
> > > XENMEM_rsrc_acq_caller_owned.
> > I am thinking to introduce p2m_mapping_owned. Or do we have a p2m_type
> > that we could re-use?
> I think we should be able to just use p2m_ram_rw if it is caller owned.
I thought about p2m_ram_rw but discarded because of the security 
implications. At least on Arm, this type can be used for foreign 
mapping, guest_copy helpers. This is not the case for p2m_mapping_foreign.
Do we want to allow thoses resources to be used in hypercall buffer 
and/or mapped by other guest via the foreign API? If not, then we want 
to use a different type.
Cheers,

--
Julien Grall

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