[PATCH] arm64: Revamp HCR_EL2.E2H RES1 detection

[Oliver Upton]

Hey,

On Thu, Oct 09, 2025 at 01:12:39PM +0100, Marc Zyngier wrote:
> We currently have two ways to identify CPUs that only implement FEAT_VHE
> and not FEAT_E2H0:
> 
> - either they advertise it via ID_AA64MMFR4_EL1.E2H0,
> - or the HCR_EL2.E2H bit is RAO/WI
> 
> However, there is a third category of "cpus" that fall between these
> two cases: on CPUs that do not implement FEAT_FGT, it is IMPDEF whether
> an access to ID_AA64MMFR4_EL1 can trap to EL2 when the register value
> is zero.
> 
> A consequence of this is that on systems such as Neoverse V2, a NV
> guest cannot reliably detect that it is in a VHE-only configuration
> (E2H is writable, and ID_AA64MMFR0_EL1 is 0), despite the hypervisor's
> best effort to repaint the id register.
> 
> Replace the RAO/WI test by a sequence that makes use of the VHE
> register remnapping between EL1 and EL2 to detect this situation,
> and work out whether we get the VHE behaviour even after having
> set HCR_EL2.E2H to 0.
> 
> This solves the NV problem, and provides a more reliable acid test
> for CPUs that do not completely follow the letter of the architecture
> while providing a RES1 behaviour for HCR_EL2.E2H.
> 
> Suggested-by: Marc Rutland <mark.rutland at arm.com>
		^~~~

Thank you *Mark* for the suggestion here, neat trick :)

I'd be in favor of this patch being sent to stable, happy to handle the
backports if you don't have the time for it. VMs mysteriously dying
isn't a very good experience on NV and I'd like to not scare folks away.

Reviewed-by: Oliver Upton <oliver.upton at linux.dev>

Thanks,
Oliver