Re: [PATCH v21 13/15] KVM: selftests: arm64: Only restore SPSR_EL1 and ELR_EL1 if they change

[Lorenzo Stoakes (ARM)]

On Wed, Sep 30, 2026 at 10:48:23PM +0100, Mark Brown wrote:
> When EXLOCK is enabled for ELx writes to SPSR_ELx and ELR_ELx generate
> GCS exceptions. Currently the selftest helpers unconditionally reload the
> saved values for these registers regardless of if the handler didn't change
> them. This gets in the way of EXLOCK testing, making it hard to generate
> contexts which will generate an invalid ERET since we never get as far as
> doing an exception return while EXLOCK is enabled.

Sounds reasonable.

>
> In order to facilitate testing of nested virtualisation emulation of EXLOCK
> change our reloads into read/modify/write cycles, suppressing the writes
> when they would not change the value in the register.

Also sounds sensible.

>
> Reviewed-by: Leonardo Bras <leo.bras@xxxxxxx>
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>

Also looks obviously correct (TM) to me so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

> ---
>  tools/testing/selftests/kvm/lib/arm64/handlers.S | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/lib/arm64/handlers.S b/tools/testing/selftests/kvm/lib/arm64/handlers.S
> index 0e443eadfac6..367c22d61cba 100644
> --- a/tools/testing/selftests/kvm/lib/arm64/handlers.S
> +++ b/tools/testing/selftests/kvm/lib/arm64/handlers.S
> @@ -33,8 +33,18 @@
>
>  .macro restore_registers
>  	ldp	x1, x2, [sp, #16 * 16] /* PC, PSTATE */
> +
> +	/* Suppress redundant writes to support GCS exception lock testing */
> +	mrs	x3, elr_el1
> +	cmp	x1, x3
> +	beq	.Lskip_elr_\@
>  	msr	elr_el1, x1
> +.Lskip_elr_\@:
> +	mrs	x3, spsr_el1
> +	cmp	x2, x3
> +	beq	.Lskip_spsr_\@

Even as an arm64 assembly novice I can see what this is doing :)

>  	msr	spsr_el1, x2
> +.Lskip_spsr_\@:
>
>  	/* sp is not restored */
>  	ldp	x30, xzr, [sp, #16 * 15] /* x30, SP */
>
> --
> 2.47.3
>
>

--
Cheers, Lorenzo