Re: [PATCH 2/2] KVM: arm64: Fix vcpu_{read,write}_sys_reg() accessors

[Oliver Upton <oliver.upton@linux.dev>]

On Sat, Aug 09, 2025 at 03:48:11PM +0100, Marc Zyngier wrote:
> @@ -144,125 +156,120 @@ static bool get_el2_to_el1_mapping(unsigned int reg,
>  		MAPPED_EL2_SYSREG(ZCR_EL2,     ZCR_EL1,     NULL	     );
>  		MAPPED_EL2_SYSREG(CONTEXTIDR_EL2, CONTEXTIDR_EL1, NULL	     );
>  		MAPPED_EL2_SYSREG(SCTLR2_EL2,  SCTLR2_EL1,  NULL	     );
> +	case CNTHCTL_EL2:
> +		/* CNTHCTL_EL2 is super special, until we support NV2.1 */
> +		loc->loc = ((is_hyp_ctxt(vcpu) && vcpu_el2_e2h_is_set(vcpu)) ?
> +			    SR_LOC_SPECIAL : SR_LOC_MEMORY);
> +		break;
> +	case TPIDR_EL0:
> +	case TPIDRRO_EL0:
> +	case TPIDR_EL1:
> +	case PAR_EL1:
> +		/* These registers are always loaded, no matter what */
> +		loc->loc = SR_LOC_LOADED;
> +		break;
>  	default:
> -		return false;
> +		/*
> +		 * Non-mapped EL2 registers are by definition in memory, but
> +		 * we don't need to distinguish them here, as the CPU
> +		 * register accessors will bail out and we'll end-up using
> +		 * the backing store.
> +		 *
> +		 * EL1 registers are, however, only loaded if we're
> +		 * not in hypervisor context.
> +		 */
> +		loc->loc = is_hyp_ctxt(vcpu) ? SR_LOC_MEMORY : SR_LOC_LOADED;

Hmm... I get the feeling that this flow is becoming even more subtle.
There's some implicit coupling between this switch statement and the
__vcpu_{read,write}_sys_reg_from_cpu() which feels like it could be
error prone. Especially since we're gonna lose the WARN() that would
inform us if an on-CPU register was actually redirected to memory.

I'm wondering if we need some macro hell containing the block of
registers we handle on-CPU and expand that can be expanded into this
triage switch case as well as the sysreg accessor.

What you have definitely seems correct, though. I'll twiddle a bit and
see if I come up with something, although I imagine what you have is
what we'll use in the end anyway.

Thanks,
Oliver