Re: [Xen-devel] [PATCH v3 0/8] xen: arm: Use super pages in p2m

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2014-06-26 at 16:03 +0100, Julien Grall wrote:
> Hi Ian,
> 
> On 06/26/2014 11:16 AM, Ian Campbell wrote:
> > The following series adds support for superpage mappings (1G and 2M) in
> > the p2m for dom0 and domU.
> > 
> > At the moment it is not possible to allocate 1G regions to guests,
> > because this is gated on multipage_allocation_permitted() which
> > currently (incorrectly) returns false for dom0 on ARM.
> 
> OOI, why multipage_allocation_permitted is gated with iomem_caps?

I don't really know.

I think it relates to the old PV x86 world where if you had a piece of
hardware passed through it might also be considered reasonable to allow
you to allocate larger contiguous buffers for the purposes of DMA (or
swiotlb) etc.

That probably doesn't make as much sense on ARM (or HVM/PVH x86 for that
matter)

> It looks wrong to me, as mapping an MMIO region in the guest will allow
> him to use 1G mapping.
> 
> > This is fixed by
> > Arianna's "arch/arm: domain build: let dom0 access I/O memory of mapped
> > devices" (which initialises iomem_caps for dom0).
> 
> IIRC, this has been pushed a month ago on upstream.

So it has. Good.

Ian.


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