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Re: [Xen-devel] grant table interface addition?

  • To: Jan Beulich <jbeulich@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
  • Date: Thu, 30 Oct 2008 14:15:17 +0000
  • Cc:
  • Delivery-date: Thu, 30 Oct 2008 07:15:40 -0700
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: Ack6me7xLYjr8KaNEd23kwAX8io7RQ==
  • Thread-topic: [Xen-devel] grant table interface addition?

On 30/10/08 14:06, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> In order to be able to set some of the available bits in the pte resulting
> from
> gnttab_set_map_op() (in particular PAGE_SPECIAL and possibly PAGE_IO),
> it would seem necessary to extend the set of flags that can be passed to
> that function. While the addition by itself is a simple operation, the
> question
> is how to deal with backward compatibility here: Is there anything
> preventing the guest kernel from setting the flags it wants manually after
> Xen established the mapping, i.e. would Xen either disallow any modification
> to such pte-s, or get confused by the pte not being identical to what it set
> it to?

I think mod_l1_entry() would allow this since it does no validation unless
the mapping or PRESENT/RW change. Direct page-table writing won't work as it
happens since it will want to get_page() which of course won't work on a
foreign page. It could be given the same fast path as mod_l1_entry(), of

> As to PAGE_IO, wouldn't it be appropriate to set this flag on pte-s created
> with GNTMAP_device_map? Or is the name misleading me? I'm perhaps
> confused by the fact that the use of this flag is not tied to granting iomem
> pages...

The idea was that this would grant-map pages into an iommu. Really it's not
actually been used, but GNTMAP_device_map doesn't cause mappings to appear
in host page tables.

 -- Keir

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