Re: [PATCH v2 01/12] KVM: s390: leave AIs in IPM of GISA during vcpu_pre_run()

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On Mon, 19 Nov 2018 18:25:25 +0100
Michael Mueller <mimu@xxxxxxxxxxxxx> wrote:

> Do not call __deliver_io() for adapter interruptions already
> pending in the IPM. That is a double effort. They will
> be processed as soon the vcpu control is given to SIE.
> 
> Signed-off-by: Michael Mueller <mimu@xxxxxxxxxxxxx>
> ---
>  arch/s390/kvm/interrupt.c | 54 ++++++++++++++++++++++-------------------------
>  1 file changed, 25 insertions(+), 29 deletions(-)

I think this patch does what it says on the tin, but I'm a bit lost as
to the why. (Might make more sense with the gib.)

Currently, we are trying to process any I/O interrupts, even if we'd
get them delivered via the gisa, when we're out of the SIE anyway.
IIRC, while this looks a bit like a belt-and-suspenders approach, it
also prevented performance problems when the vcpu did not go back into
the SIE immediately (it even may exit to userspace).

Also, if you're ignoring the I/O interrupts pending in the ipm, you may
end up delivering interrupts with a lower priority (higher isc) first.
I'm not sure that's what we want.

But maybe I'm just missing another bit of the code that makes this
safe. Can you elaborate a bit?



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