Re: [PATCH v4 1/6] KVM: arm64: Correctly treat writes to OSLSR_EL1 as undefined

[Oliver Upton <oupton@google.com>]

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
> | 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 :-)

> With that:
> 
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Thanks!

--
Best,
Oliver