Re: [Xen-devel] IOREQ server on Arm

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Sep 26, 2018 at 11:32:38AM +0100, Julien Grall wrote:
> Hi,
> 
> On 09/26/2018 10:14 AM, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > > Sent: 26 September 2018 09:09
> > > To: Julien Grall <julien.grall@xxxxxxx>; 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
> > > 
> > > > > > 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.
> > > 
> > 
> > Yes, that's correct. There is a FIXME clause in acquire_resource so that 
> > that only caller owned resources can be mapped by HVM/PVH domains. Thus the 
> > new call can be used for IOREQ server pages, but not grant frames.
> 
> I don't understand your last sentence. IOREQ is caller owned and therefore
> should work.
> 
> As I said, I don't have any problem with mapping the resource. The problem
> is when unmapping it because the region is set with p2m_mapping_foreign. You
> will reach the ASSERT(fdom != p2m->domain) in p2m_get_page_from_gfn.
> 
> Regardless the reference problem (we support it on Arm). Can you explain how
> this is working on PVH Dom0 today?

IOREQs (QEMU) used to work on a PVH Dom0 in the past, it's been some
time since I've launched a HVM guest now, but it definitely worked
before, like it worked for PVHv1 Dom0.

Roger.

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