Re: [PATCH 1/2] KVM: arm64: Initialize HCR_EL2.E2H early

[Leo Yan <leo.yan@arm.com>]

On Fri, Feb 28, 2025 at 09:43:20AM +0000, Marc Zyngier wrote:

[...]

> On Fri, 28 Feb 2025 09:29:55 +0000,
> Leo Yan <leo.yan@arm.com> wrote:
> >
> > Hi Mark,
> >
> > On Thu, Feb 27, 2025 at 06:05:25PM +0000, Mark Rutland wrote:
> >
> > [...]
> >
> > > +.macro init_el2_hcr        val
> > > +   mov_q   x0, \val
> > > +
> > > +   /*
> > > +    * Compliant CPUs advertise their VHE-onlyness with
> > > +    * ID_AA64MMFR4_EL1.E2H0 < 0. On such CPUs HCR_EL2.E2H is RES1, but it
> > > +    * can reset into an UNKNOWN state and might not read as 1 until it has
> > > +    * been initialized explicitly.
> >
> > For ID_AA64MMFR4_EL1.E2H0 < 0 case, the code actually clears the
> > HCR_EL2.E2H bit.
> >
> > Hence, the comment should be corrected as: "... it can reset into an
> > UNKNOWN state and might not read as 0 until it has been initialized
> > explicitly".
> 
> The comment is just fine. It is the code that is wrong, as it avoids
> setting E2H when E2H0 < 0 while we want the exact opposite behaviour.
> 
> As a result, 'b.lt' really should be a 'b.ge'. Or the original code
> kept as is.

Thanks for correcting.

> > > +    *
> > > +    * Fruity CPUs seem to have HCR_EL2.E2H set to RAO/WI, but
> > > +    * don't advertise it (they predate this relaxation).
> > > +    *
> > > +    * Initalize HCR_EL2.E2H so that later code can rely upon HCR_EL2.E2H
> > > +    * indicating whether the CPU is running in E2H mode.
> > > +    */
> >
> > I think it is even better to clear the HCR_E2H bit first. This can
> > avoid any dependency on the passed parameter 'val'.
> 
> What are you trying to avoid? A random value passed as a parameter to
> the macro?

Yes, an unexpected value passed to the macro is a concern.

If the intentaion here is only to set the HCR_EL2.E2H bit when E2H0 < 0,
we don't need to cosndier other configurations and current code is just
fine?

Thanks,
Leo