Re: [Xen-devel] [PATCH] xen/arm64: ensure that the correct SP is used for exceptions

[Julien Grall <julien.grall@xxxxxxx>]

(CC Wei for release-ack)

Hello Kyle,

On 28/04/16 18:14, Kyle Temkin wrote:
> From: "Kyle J. Temkin" <temkink@xxxxxxxxxxxx>
>
> The ARMv8 architecture has a SPSel ("stack pointer selection") machine
> register that allows us to determine which exception level's stack
> pointer is loaded when an exception occurs. As we don't want to
> use the non-priveleged SP_EL0 stack pointer -- or even assume that SP_EL0
NIT: s/priveleged/privileged/

> points to a valid address in the hypervisor context--  we'll need to ensure
> that our EL2 code sets the SPSel to SP_ELn mode, so exceptions that trap
> to EL2 use the EL2 stack pointer.
>
> This corrects an issue that can manifest as a hang-on-IRQ on some
> arm64 cores if the firmware/bootloader has previously initialized SPSel
> to 0; in which case Xen's exceptions will incorrectly use an invalid SP_EL0,
> and will endlessly spin on the synchronous abort handler.
>
> Signed-off-by: Kyle Temkin <temkink@xxxxxxxxxxxx>
Reviewed-by: Julien Grall <julien.grall@xxxxxxx>

Wei, this is a bug-fix and I think it should go to Xen 4.7.

We would also need to backport this patch on Xen 4.4 -> Xen 4.6.

Regards,

> ---
>   xen/arch/arm/arm64/head.S | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 946e2c9..d5831f2 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -361,6 +361,11 @@ skip_bss:
>           ldr   x0, =(HSCTLR_BASE)
>           msr   SCTLR_EL2, x0
>
> +        /* Ensure that any exceptions encountered at EL2
> +         * are handled using the EL2 stack pointer, rather
> +         * than SP_EL0. */
> +        msr spsel, #1
> +
>           /* Rebuild the boot pagetable's first-level entries. The structure
>            * is described in mm.c.
>            *
--
Julien Grall

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