[PATCH 3/5] arm64: Detect disabled HVC early

[Will Deacon]

On Thu, Aug 12, 2021 at 08:02:11PM +0100, Marc Zyngier wrote:
> Having HVC disabled from EL3 while the kernel is entered at EL2
> is a complete nightmare.
> 
> We end-up taking an UNDEF at the worse possible moment (checking
> for VHE) and even if we didn't, having KVM enabled would signify
> the premature end of the kernel.
> 
> Instead, try and detect this stupid case by issuing a HVC
> for HVC_RESET_VECTORS, which does nothing when the stubs
> are live. If we get HVC_STUB_ERR back, that's because the
> UNDEF handler has kicked in.
> 
> In this situation, close your eyes, block your nose, and gracefully
> pretend we have booted at EL1.
> 
> Reported-by: Rafał Miłecki <zajec5 at gmail.com>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kernel/head.S | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 9d5aa56a98cc..d6b2b05f5d3a 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -519,6 +519,28 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
>  	msr	vbar_el2, x0
>  	isb
>  
> +	// Check that HVC actually works...
> +	mov	x0, #HVC_RESET_VECTORS
> +	hvc	#0
> +
> +	mov_q	x1, HVC_STUB_ERR
> +	cmp	x0, x1
> +	b.ne	0f
> +
> +	/*
> +	 * HVC is unusable, so pretend we actually booted at EL1.
> +	 * Once we have left EL2, there will be no going back.
> +	 */

This comment got me thinking...

	.macro	host_hvc0
	mrs	xzr, actlr_el1
	.endm

then set HCR_EL2.TACR=1 while we still can and match the ISS against a
constant in the handler. Too awful?

Will