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

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

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.

Roger.

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