Re: [Xen-devel] Xen: ARM: Support for mapping ECAM PCIe Config Space Specified In Static ACPI Table

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 03/01/17 19:27, Stefano Stabellini wrote:
> On Wed, 28 Dec 2016, Julien Grall wrote:
> > On 20/12/16 22:33, Stefano Stabellini wrote:
> > > On Tue, 20 Dec 2016, Julien Grall wrote:
> > > > On 20/12/2016 00:54, Stefano Stabellini wrote:
> > > > > On Mon, 19 Dec 2016, Julien Grall wrote:
> > > > > > On 16/12/2016 15:49, Julien Grall wrote:
> > > > > > > On 14/12/16 08:00, Jiandi An wrote:
> > I disagree here. You make the assumption that DOM0 will always issue the
> > hypercall with MFN == GFN.
> >
> > Today we only check whether the region is allowed in iomem. If it is not
> > allowed we will ignore the request and report as "succeeded". But it does not
> > mean there will be an emulation behind nor DOM0 decided to map the region with
> > MFN != GFN.
> >
> > So DOM0 expects the region to be mapped, but actually it may not be done by
> > the hypervisor. It will be a pain to debug it because the error may come up
> > much later.
> >
> > The description of the hypercall is "the region is mapped in Stage-2 using the
> > memory attribute Device-nGnRE". Nowhere it is stated the region will not be
> > mapped if emulated nor must be called MFN == GFN.
> >
> > Now, we have two hypercalls XEN_DOMCTL_memory_mapping and
> > XENMEM_add_to_physmap_range having two distinct behavior when mapping an MMIO
> > into a guest.
> >
> > We should at least return an error, even if DOM0 decides to ignore it.
> >
> > I am open to any other suggestion. But I don't think the hypercall should
> > silently ignore a request as it is done today.
> I agree that assigning clear and unequivocal error codes is a good idea.
> The error code for "failure to map" should be different from the error
> code for "this belongs to an emulated device".
This hypercall is part of the stable ABI. So would it be fine to modify 
the error code?
This hypercall was introduced in Xen 4.7 and used with ACPI. The current 
behavior of Linux is to print a warning if the error code is not 0 and 
then return success. So previous Linux version would print spurious warning.
ACPI support for ARM in the hypervisor is still marked as experimental. 
I would like to see it fully supported in Xen 4.9. Before that, I think 
we should fix the hypercall.
Cheers,

--
Julien Grall

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