From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
George Dunlap <george.dunlap@citrix.com>,
Ian Jackson <iwj@xenproject.org>,
Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
Luca Fancellu <luca.fancellu@arm.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH] public/gnttab: relax v2 recommendation
Date: Fri, 30 Apr 2021 10:36:06 +0200 [thread overview]
Message-ID: <71e61d09-5d92-94dc-ae0c-ce09cb49b4ce@suse.com> (raw)
In-Reply-To: <8eac6f09-4d1d-6fcc-4218-8c9a0760a6bb@xen.org>
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@suse.com>
>>
>> --- 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.
Jan
next prev parent reply other threads:[~2021-04-30 8:36 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-29 13:10 [PATCH] public/gnttab: relax v2 recommendation Jan Beulich
2021-04-30 7:42 ` Luca Fancellu
2021-04-30 8:19 ` Julien Grall
2021-04-30 8:36 ` Jan Beulich [this message]
2021-05-05 8:12 ` Julien Grall
2021-05-05 8:24 ` Jan Beulich
2021-05-05 8:51 ` Julien Grall
2021-05-05 10:57 ` Jan Beulich
2021-05-05 11:50 ` Julien Grall
2021-05-05 12:18 ` Jan Beulich
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