[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [future abi] [RFC PATCH V3] xen/gnttab: Store frame GFN in struct page_info on Arm

Hi Roger,

On 24/09/2021 13:41, Roger Pau Monné wrote:
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:

! 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.

Or you could find a hole that is outside of the RAM regions. This is not trivial but not impossible (see [1]).

Other question is whether we want/need to keep such mode going

I am assunming by "such mode" you mean "identity mapped". If so, then I am afraid this is not going to disappear on Arm at least. There are still out there many platforms without IOMMUs or devices which are not protected (the GPU is a common one).

Furthermore, Arm just sent a series to introduce identity mapping for domUs as well (see [2]).

[1] <1631034578-12598-1-git-send-email-olekstysh@xxxxxxxxx>
[2] <20210923031115.1429719-1-penny.zheng@xxxxxxx>


Julien Grall



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.