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Date:   Wed, 15 Dec 2021 14:32:04 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Oliver Upton <oupton@google.com>
Cc:     kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
        Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 1/6] KVM: arm64: Correctly treat writes to OSLSR_EL1
 as undefined
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On Wed, Dec 15, 2021 at 01:09:28PM +0000, Oliver Upton wrote:
> Hi Mark,
> 
> On Wed, Dec 15, 2021 at 11:39:58AM +0000, Mark Rutland wrote:
> > Hi Oliver,
> > 
> > On Tue, Dec 14, 2021 at 05:28:07PM +0000, Oliver Upton wrote:
> > > Any valid implementation of the architecture should generate an
> > > undefined exception for writes to a read-only register, such as
> > > OSLSR_EL1. Nonetheless, the KVM handler actually implements write-ignore
> > > behavior.
> > > 
> > > Align the trap handler for OSLSR_EL1 with hardware behavior. If such a
> > > write ever traps to EL2, inject an undef into the guest and print a
> > > warning.
> > 
> > I think this can still be read amibguously, since we don't explicitly state
> > that writes to OSLSR_EL1 should never trap (and the implications of being
> > UNDEFINED are subtle). How about:
> > 
> > | Writes to OSLSR_EL1 are UNDEFINED and should never trap from EL1 to EL2, but
> > | the KVM trap handler for OSLSR_EL1 handlees writes via ignore_write(). This

Whoops, with s/handlees/handles/

> > | is confusing to readers of the code, but shouldn't have any functional impact.
> > |
> > | For clarity, use write_to_read_only() rather than ignore_write(). If a trap
> > | is unexpectedly taken to EL2 in violation of the architecture, this will
> > | WARN_ONCE() and inject an undef into the guest.
> 
> Agreed, I like your suggested changelog better :-)

Cool!

Mark.

> 
> > With that:
> > 
> > Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> 
> Thanks!
> 
> --
> Best,
> Oliver