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Re: [PATCH] xen/vcpu: remove vcpu_set_singleshot_timer flags field
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 18 Apr 2023 18:02:30 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Henry Wang <Henry.Wang@xxxxxxx>, Community Manager <community.manager@xxxxxxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Tue, 18 Apr 2023 16:03:02 +0000
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On Tue, Apr 18, 2023 at 04:54:49PM +0100, Andrew Cooper wrote:
> 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.
I agree, but didn't think it was necessary to get into debating
whether it's useful or not, since its introduction was bogus anyway.
> 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.
Oh, I think I've dropped the comment I had added next to
VCPU_SSHOTTMR_future that contained /* Ignored. */ (just like for the whole
flags field).
I can elaborate a bit on why VCPU_SSHOTTMR_future is not useful in the
commit log, and add that Ignored comment to the flag.
Thanks, Roger.
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