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Date: Tue, 6 Apr 2021 14:17:22 +1000
From: Paul Mackerras <paulus@ozlabs.org>
To: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v6 09/48] powerpc/64s: remove KVM SKIP test from
 instruction breakpoint handler
Message-ID: <YGvg0jCz85Uev3cm@thinks.paulus.ozlabs.org>
References: <20210405011948.675354-1-npiggin@gmail.com>
 <20210405011948.675354-10-npiggin@gmail.com>
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Cc: Fabiano Rosas <farosas@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org,
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On Tue, Apr 06, 2021 at 12:44:45PM +1000, Nicholas Piggin wrote:
> Excerpts from Nicholas Piggin's message of April 5, 2021 11:19 am:
> > The code being executed in KVM_GUEST_MODE_SKIP is hypervisor code with
> > MSR[IR]=0, so the faults of concern are the d-side ones caused by access
> > to guest context by the hypervisor.
> > 
> > Instruction breakpoint interrupts are not a concern here. It's unlikely
> > any good would come of causing breaks in this code, but skipping the
> > instruction that caused it won't help matters (e.g., skip the mtmsr that
> > sets MSR[DR]=0 or clears KVM_GUEST_MODE_SKIP).
> > 
> >  [Paul notes: the 0x1300 interrupt was dropped from the architecture a
> >   long time ago and is not generated by P7, P8, P9 or P10.]
> > 
> > In fact it does not exist in ISA v2.01, which is the earliest supported
> > now, but did exist in 600 series designs (some of the earliest 64-bit
> > powerpcs), so it could probably be removed entirely.
> 
> Hmm, I looked at a 970 manual and that does have a 0x1300, but 2.01 
> (which it implements) does not, if I'm reading correctly. Seems strange
> the interrupt would be taken out of the architecture then implemented,
> but not mine to wonder why. Maybe I misread something.

It was probably put in at Apple's request, since they had been using
IABR and the 0x1300 interrupt on the 6xx/7xx/7xxx CPUs.

Paul.