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Re: [PATCH] xen/vcpu: remove vcpu_set_singleshot_timer flags field
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 18 Apr 2023 16:54:49 +0100
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On 18/04/2023 4:42 pm, Roger Pau Monne wrote:
> The addition of the flags field in the vcpu_set_singleshot_timer in
> 505ef3ea8687 is an ABI breakage, as the size of the structure is
> increased.
>
> Remove such field addition and drop the implementation of the
> VCPU_SSHOTTMR_future flag. If a timer provides an expired timeout
> value just inject the timer interrupt.
>
> Bump the Xen interface version, and keep the flags field and
> VCPU_SSHOTTMR_future available for guests using the old interface.
>
> Note the removal of the field from the vcpu_set_singleshot_timer
> struct allows removing the compat translation of the struct.
>
> Fixes: 505ef3ea8687 ('Add flags field to VCPUOP_set_singlsehot_timer.')
> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
While everything said is true, this isn't the reason to to get rid of
VCPU_SSHOTTMR_future
It 505ef3ea8687 does appear to have been an ABI break, that's
incidental. It dates from 2007 so whatever we have now is the de-facto
ABI, whether we like it or not.
The reason to delete this is because it is a monumentality and entirely
stupid idea which should have been rejected outright at the time, and
the only guest we can find which uses it also BUG_ON()'s in response to
-ETIME.
It can literally only be used to shoot yourself in the foot with, and
more recent Linuxes have dropped their use of it.
The structure needs to stay it's current shape, and while it's fine to
hide the VCPU_SSHOTTMR_future behind an interface version macro, we do
need to say that it is explicitly ignored.
~Andrew
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