Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling controls to EL2

Andrew Murray
2020-01-09T17:46:27Z

On Thu, Jan 09, 2020 at 05:42:51PM +0000, Mark Rutland wrote:
> Hi Andrew,
>
> On Thu, Jan 09, 2020 at 05:25:12PM +0000, Andrew Murray wrote:
> > On Mon, Dec 23, 2019 at 12:10:42PM +0000, Andrew Murray wrote:
> > > On Mon, Dec 23, 2019 at 12:05:12PM +0000, Marc Zyngier wrote:
> > > > On 2019-12-23 11:56, Andrew Murray wrote:
> > > > > My original concern in the cover letter was in how to prevent
> > > > > the guest from attempting to use these registers in the first
> > > > > place - I think the solution I was looking for is to
> > > > > trap-and-emulate ID_AA64DFR0_EL1 such that the PMSVer bits
> > > > > indicate that SPE is not emulated.
> > > >
> > > > That, and active trapping of the SPE system registers resulting in injection
> > > > of an UNDEF into the offending guest.
> > >
> > > Yes that's no problem.
> >
> > The spec says that 'direct access to [these registers] are UNDEFINED' - is it
> > not more correct to handle this with trap_raz_wi than an undefined instruction?
>
> The term UNDEFINED specifically means treated as an undefined
> instruction. The Glossary in ARM DDI 0487E.a says for UNDEFINED:
>
> | Indicates cases where an attempt to execute a particular encoding bit
> | pattern generates an exception, that is taken to the current Exception
> | level, or to the default Exception level for taking exceptions if the
> | UNDEFINED encoding was executed at EL0. This applies to:
> |
> | * Any encoding that is not allocated to any instruction.
> |
> | * Any encoding that is defined as never accessible at the current
> | Exception level.
> |
> | * Some cases where an enable, disable, or trap control means an
> | encoding is not accessible at the current Exception level.
>
> So these should trigger an UNDEFINED exception rather than behaving as
> RAZ/WI.

OK thanks for the clarification - I'll leave it as an undefined instruction.

Thanks,

Andrew Murray

>
> Thanks,
> Mark.