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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.
Cheers,
--
Julien Grall
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