From: Eric Farman <farman@linux.ibm.com>
To: Christian Borntraeger <borntraeger@linux.ibm.com>,
David Hildenbrand <david@redhat.com>,
Janosch Frank <frankja@linux.ibm.com>,
Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: kvm@vger.kernel.org, linux-s390@vger.kernel.org,
Heiko Carstens <hca@linux.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Alexander Gordeev <agordeev@linux.ibm.com>,
Sven Schnelle <svens@linux.ibm.com>
Subject: Re: [PATCH] KVM: s390: vsie: retry SIE instruction on host intercepts
Date: Mon, 04 Mar 2024 10:37:29 -0500 [thread overview]
Message-ID: <8fbd41c0fb16a5e10401f6c2888d44084e9af86a.camel@linux.ibm.com> (raw)
In-Reply-To: <1deb0e32-7351-45d2-a342-96a659402be8@linux.ibm.com>
On Mon, 2024-03-04 at 09:44 +0100, Christian Borntraeger wrote:
>
>
> Am 04.03.24 um 09:35 schrieb David Hildenbrand:
> > On 01.03.24 21:43, Eric Farman wrote:
> > > It's possible that SIE exits for work that the host needs to
> > > perform
> > > rather than something that is intended for the guest.
> > >
> > > A Linux guest will ignore this intercept code since there is
> > > nothing
> > > for it to do, but a more robust solution would rewind the PSW
> > > back to
> > > the SIE instruction. This will transparently resume the guest
> > > once
> > > the host completes its work, without the guest needing to process
> > > what is effectively a NOP and re-issue SIE itself.
> >
> > I recall that 0-intercepts are valid by the architecture. Further,
> > I recall that there were some rather tricky corner cases where
> > avoiding 0-intercepts would not be that easy.
Any chance you recall any details of those corner cases? I can try to
chase some of them down.
> >
> > Now, it's been a while ago, and maybe I misremember. SoI'm trusting
> > people with access to documentation can review this.
>
> Yes, 0-intercepts are allowed, and this also happens when LPAR has an
> exit.
From an offline conversation I'd had some months back:
"""
The arch does allow ICODE=0 to be stored, but it's supposed to happen
only upon a host interruption -- in which case the old PSW is supposed
to point back at the SIE, to resume guest execution if the host should
LPSW oldPSW.
"""
> So this patch is not necessary, the question is if this would be an
> valuable optimization?
It's a reasonable question. I don't think I have a reasonable way of
measuring the exit, though. :/
next prev parent reply other threads:[~2024-03-04 15:37 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-01 20:43 [PATCH] KVM: s390: vsie: retry SIE instruction on host intercepts Eric Farman
2024-03-04 8:35 ` David Hildenbrand
2024-03-04 8:44 ` Christian Borntraeger
2024-03-04 15:37 ` Eric Farman [this message]
2024-04-29 10:18 ` Christian Borntraeger
2024-04-30 19:31 ` Eric Farman
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