[PATCH v8 25/38] KVM: arm64: Trap SME usage in guest

[Mark Brown]

On Tue, Jan 25, 2022 at 11:27:55AM +0000, Marc Zyngier wrote:
> Mark Brown <broonie at kernel.org> wrote:

> > +	if (IS_ENABLED(CONFIG_ARM64_SME) && cpus_have_final_cap(ARM64_SME))

> Please drop the IS_ENABLED(). We purposely avoid conditional
> compilation in KVM in order to avoid bitrot, and the amount of code
> you save isn't significant. Having a static key is more than enough to
> avoid runtime costs.

Sure, I wanted to be extra careful here as this is all in hot paths and
going to get moved elsewhere when we have real guest support.

> > +	if (IS_ENABLED(CONFIG_ARM64_SME) && cpus_have_final_cap(ARM64_SME) &&
> > +	    cpus_have_final_cap(ARM64_HAS_FGT)) {
> > +		val = read_sysreg_s(SYS_HFGRTR_EL2);
> > +		val &= ~(HFGxTR_EL2_nTPIDR_EL0_MASK |
> > +			 HFGxTR_EL2_nSMPRI_EL1_MASK);
> > +		write_sysreg_s(val, SYS_HFGRTR_EL2);
> > +
> > +		val = read_sysreg_s(SYS_HFGWTR_EL2);
> > +		val &= ~(HFGxTR_EL2_nTPIDR_EL0_MASK |
> > +			 HFGxTR_EL2_nSMPRI_EL1_MASK);
> > +		write_sysreg_s(val, SYS_HFGWTR_EL2);
> > +	}

> If the CPUs do not have FGT, what provides the equivalent trapping?

Nothing for nVHE mode.

> If FGT is mandatory when SME exists, then you should simplify the
> condition.

OK, I'll remove the defensiveness here.  FGT is mandatory from v8.6 and
SME is a v9 feature so people shouldn't build a SME implementation that
lacks FGT.
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