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Re: [PATCH 4/9] gnttab: drop GNTMAP_can_fail

  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 26 Aug 2021 15:03:56 +0200
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  • Cc: George Dunlap <george.dunlap@xxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 26 Aug 2021 13:04:06 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 26.08.2021 13:45, Andrew Cooper wrote:
> On 26/08/2021 11:13, Jan Beulich wrote:
>> There's neither documentation of what this flag is supposed to mean, nor
>> any implementation. With this, don't even bother enclosing the #define-s
>> in a __XEN_INTERFACE_VERSION__ conditional, but drop them altogether.
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> It was introduced in 4d45702cf0398 along with GNTST_eagain, and the
> commit message hints that it is for xen-paging
> Furthermore, the first use of GNTST_eagain was in ecb35ecb79e0 for
> trying to map/copy a paged-out frame.
> Therefore I think it is reasonable to conclude that there was a planned
> interaction between GNTMAP_can_fail and paging, which presumably would
> have been "don't pull this in from disk if it is paged out".

I suppose that's (far fetched?) guesswork.

> I think it is fine to drop, as no implementation has ever existed in
> Xen, but it would be helpful to have the history summarised in the
> commit message.

I've extended it to

"There's neither documentation of what this flag is supposed to mean, nor
 any implementation. Commit 4d45702cf0398 ("paging: Updates to public
 grant table header file") suggests there might have been plans to use it
 for interaction with mem-paging, but no such functionality has ever
 materialized. With this, don't even bother enclosing the #define-s in a
 __XEN_INTERFACE_VERSION__ conditional, but drop them altogether."




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