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

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

On Mon, Sep 18, 2017 at 06:32:22PM +0100, Julien Grall wrote:
> 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).

IIRC this only applies to auto-translated (HVM) domains, not to PV
domains. QEMU stubdomains are PV guests.

Roger.

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