From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <george.dunlap@citrix.com>,
Ian Jackson <iwj@xenproject.org>, Julien Grall <julien@xen.org>,
Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 3/3] gnttab: GTF_sub_page is a v2-only flag
Date: Thu, 18 Feb 2021 16:02:14 +0100 [thread overview]
Message-ID: <a1a0587e-3e20-a023-6dd0-3b37b34fdf17@suse.com> (raw)
In-Reply-To: <e00d9bed-20e4-b5a5-91e8-02f7a6689f86@citrix.com>
On 18.02.2021 15:22, Andrew Cooper wrote:
> On 17/02/2021 10:46, Jan Beulich wrote:
>> Prior to its introduction, v1 entries weren't checked for this flag, and
>> the flag also has been meaningless for v1 entries. Therefore it also
>> shouldn't be checked. (The only consistent alternative would be to also
>> check for all currently undefined flags to be clear.)
>
> We recently had a similar discussion for the stable libs.
>
> Whatever we do, an unexpected corner case needs to break. Checking for
> all undefined flags up front is far cleaner - absolutely nothing good
> can come for a guest which set GTF_sub_page with v1, and is expecting it
> to work, and this way, we do all breaking in one go, rather than
> breaking $N times in the future as new flags get added.
Except that there doesn't need to be any breaking: v1 could continue
to ignore all originally undefined flags. v1 and v2 could continue to
ignore all flags presently undefined. See also Julien's question
about GTF_sub_page on a transitive grant entry. That's presently an
ignored setting as well (and, as an implementation detail, in fact
getting forced to be set, but with a range covering the entire page).
Retroactively starting to check (and reject) undefined flags isn't
going to be nice; nevertheless I wanted to put this up as an at
least theoretical alternative. Perhaps a topic for the next community
call, if I don't forget by the time an agenda page appears.
Jan
prev parent reply other threads:[~2021-02-18 15:02 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-17 10:42 [PATCH 0/3] gnttab: misc fixes Jan Beulich
2021-02-17 10:46 ` [PATCH 1/3] gnttab: never permit mapping transitive grants Jan Beulich
2021-02-18 10:25 ` Julien Grall
2021-02-18 11:31 ` Jan Beulich
2021-02-17 10:46 ` [PATCH 2/3] gnttab: bypass IOMMU (un)mapping when a domain is (un)mapping its own grant Jan Beulich
2021-02-17 11:03 ` Julien Grall
2021-02-17 11:38 ` Jan Beulich
2021-02-17 11:41 ` Julien Grall
2021-02-17 13:16 ` Jan Beulich
2021-02-17 14:27 ` Julien Grall
2021-02-17 10:46 ` [PATCH 3/3] gnttab: GTF_sub_page is a v2-only flag Jan Beulich
2021-02-18 14:22 ` Andrew Cooper
2021-02-18 15:02 ` Jan Beulich [this message]
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