Re: [PATCH] KVM: arm64: Preserve all guest ZCR_EL2.LEN values

[Mark Brown]

On Sat, May 23, 2026 at 09:47:38AM +0100, Marc Zyngier wrote:
> Mark Brown <broonie@xxxxxxxxxx> wrote:

> > The reasoning for the current behaviour is not specifically articulated, my
> > best guess is that it is intended to ensure that the guest can not see an
> > effective VL greater than the maximum that has been configured. This can
> > instead be achieved by configuring ZCR_EL2 when loading guest state:

> >  - When running at EL0 or EL1 configure ZCR_EL2.LEN to the minimum of the
> >    guest ZCR_EL2.LEN and vcpu_sve_max_vq(vcpu)-1.

> This is not EL0 or EL1. This is when in a nested context (i.e. running
> a L2 guest), as EL0 exists for L1 as well.

Sorry, this was intended to be specifically for a L2 guest but didn't
actually say that.  I originally had more verbosity in the commit log
that I cleaned up too much, making things unclear.  I will clarify.

> > Currently all other bits in ZCR_EL2 are either RES0 or RAZ/WI, values
> > written are sanitised based on this.

> Only for the direct writes to ZCR_EL2, as they are trapping. I don't
> see any sanitisation for writes using the ZCR_EL1 accessor, which is
> the common case. This needs fixing at the same time.

OK, I'll convert ZCR_EL2 to a sanitised register.  As I mentioned I was
a bit confused about why the existing code is the way it is and so
followed it in only managing the direct writes.  I figured it was
considered OK to rely on the hardware for the RES0 and WI behaviour for
untrapped access.

> > -		if (is_nested_ctxt(vcpu)) -			zcr_el2
> > = __vcpu_sys_reg(vcpu, ZCR_EL2); -		else -
> > zcr_el2 = vcpu_sve_max_vq(vcpu) - 1; +		if
> > (is_nested_ctxt(vcpu) && !is_hyp_ctxt(vcpu)) +
> > zcr_el2 = min(zcr_el2, __vcpu_sys_reg(vcpu, ZCR_EL2));

> Why the change in the condition guarding this? Given the definition of
> is_nested_ctxt(), this seems unnecessary.

You're right, this change is not needed.  I had misremembered what
is_nested_ctxt() was checking.

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