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Re: [future abi] [RFC PATCH V3] xen/gnttab: Store frame GFN in struct page_info on Arm
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Fri, 24 Sep 2021 10:41:51 +0200
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- Cc: Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, "Ian Jackson" <iwj@xxxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Fri, 24 Sep 2021 08:42:10 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Thu, Sep 23, 2021 at 09:59:26PM +0100, Andrew Cooper wrote:
> On 23/09/2021 20:32, Oleksandr Tyshchenko wrote:
> > Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
> > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > ---
> > You can find the related discussions at:
> > https://lore.kernel.org/xen-devel/93d0df14-2c8a-c2e3-8c51-54412190171c@xxxxxxx/
> > https://lore.kernel.org/xen-devel/1628890077-12545-1-git-send-email-olekstysh@xxxxxxxxx/
> > https://lore.kernel.org/xen-devel/1631652245-30746-1-git-send-email-olekstysh@xxxxxxxxx/
> >
> > ! Please note, there is still unresolved locking question here for which
> > I failed to find a suitable solution. So, it is still an RFC !
>
> Just FYI, I thought I'd share some of the plans for ABI v2. Obviously
> these plans are future work and don't solve the current problem.
>
> Guests mapping Xen pages is backwards. There are reasons why it was
> used for x86 PV guests, but the entire interface should have been design
> differently for x86 HVM.
>
> In particular, Xen should be mapping guest RAM, rather than the guest
> manipulating the 2nd stage tables to map Xen RAM. Amongst other things,
> its far far lower overhead.
>
>
> A much better design is one where the grant table looks like an MMIO
> device. The domain builder decides the ABI (v1 vs v2 - none of this
> dynamic switch at runtime nonsense), and picks a block of guest physical
> addresses, which are registered with Xen. This forms the grant table,
> status table (v2 only), and holes to map into.
I think this could be problematic for identity mapped Arm dom0, as
IIRC in that case grants are mapped so that gfn == mfn in order to
account for the lack of an IOMMU. You could use a bounce buffer, but
that would introduce a big performance penalty.
Other question is whether we want/need to keep such mode going
forward.
Regards, Roger.
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