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

Re: [PATCH] public/gnttab: relax v2 recommendation

Hi Jan,

On 30/04/2021 09:36, Jan Beulich wrote:
On 30.04.2021 10:19, Julien Grall wrote:
On 29/04/2021 14:10, Jan Beulich wrote:
With there being a way to disable v2 support, telling new guests to use
v2 exclusively is not a good suggestion.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/public/grant_table.h
+++ b/xen/include/public/grant_table.h
@@ -121,8 +121,10 @@ typedef uint32_t grant_ref_t;
- * Version 1 of the grant table entry structure is maintained purely
- * for backwards compatibility.  New guests should use version 2.
+ * Version 1 of the grant table entry structure is maintained largely for
+ * backwards compatibility.  New guests are recommended to support using
+ * version 2 to overcome version 1 limitations, but to be able to fall back
+ * to version 1.

v2 is not supported on Arm and I don't see it coming anytime soon.
AFAIK, Linux will also not use grant table v2 unless the guest has a
address space larger than 44 (?) bits.

Yes, as soon as there are frame numbers not representable in 32 bits.

I can't remember why Linux decided to not use it everywhere, but this is
a sign that v2 is not always desired.

So I think it would be better to recommend new guest to use v1 unless
they hit the limitations (to be details).

IOW you'd prefer s/be able to fall back/default/? I'd be fine that way

Yes. We would also need to document the limitations as they don't seem to be (clearly?) written down in the headers.


Julien Grall



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