Re: [Xen-devel] [PATCH v2] xen: grant-table: Simplify get_paged_frame

[Julien Grall <julien.grall@xxxxxxx>]

Hi Roger,

On 18/09/17 17:58, Roger Pau Monné wrote:
> On Mon, Sep 18, 2017 at 05:27:52PM +0100, Julien Grall wrote:
> > The implementation of get_paged_frame is currently different whether the
> > architecture support sharing memory or paging memory. Both
> > version are extremely similar so it is possible to consolidate in a
> > single implementation.
> >
> > The main difference is the x86 version will allow grant on foreign page
> > when using HVM/PVH whilst Arm does not. At the moment, on x86 foreign pages
> > are only allowed for PVH Dom0. It seems that foreign pages should never
> > be granted so deny them
> >
> > The check for shared/paged memory are now gated with the respective ifdef.
> > Potentially, dummy p2m_is_shared/p2m_is_paging could be implemented for
> > Arm.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> >
> > ---
> >
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Cc: Tim Deegan <tim@xxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> >
> >      Changes in v2:
> >          - Deny grant on foreign page (aligned with the ARM code)
> >          - Use #ifdef rather than #if defined
> >          - Update commit message
> >          - Fix typo in the title
> >
> > get_page_from_gfn will be able to get reference on foreign page and as
> > per my understanding will allow to grant page on foreign memory.
> >
> > This was not allowed with a simple get_page(...) on the ARM
> > implementation (no sharing nor paging supprot) but is allowed on the x86
> > implementation due to get_page_from_gfn.
> >
> > On x86, foreign pages are currently only allowed for PVH dom0, so I
> > think it is not a big deal for now.
> >
> > On Arm, foreign pages can be present on any domain. So this patch would
> > permit grant on foreing pages.
> >
> > This patch will deny granting foreign pages. Jan Beulich is happy with
> > it. Any other opinions?
> Won't this break QEMU running in stub domains?
>
> I haven't tested it, but I'm afraid QEMU running in a stub domain
> might try to grant a foreign frame. Ie: the emulated network code in
> QEMU might try to grant a foreign frame in order to forward operations
> from emulated devices to PV frontends.
I don't think it will break any existing setup because foreign mapping 
are only allowed for the hardware domain (see p2m_add_foreign).
Now, what should be expected in the future? I am not sure why we would 
allow grant on foreign mapping. It is very similar to QEMU granting a 
page that was granted by another domain, this is not allowed today.
Cheers,

--
Julien Grall

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