Re: [Xen-devel] [PATCH v3 01/19] xen/arm: Save ESR_EL2 to avoid using mismatched value in syndrome check

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 31/03/17 14:07, Wei Chen wrote:
> Xen will do exception syndrome check while some types of exception
> take place in EL2. The syndrome check code read the ESR_EL2 register
> directly, but in some situation this register maybe overridden by
> nested exception.
>
> For example, if we re-enable IRQ before reading ESR_EL2 which means
> Xen may enter in IRQ exception mode and return the processor with
> clobbered ESR_EL2 (See ARM ARM DDI 0487A.j D7.2.25)
>
> In this case the guest exception syndrome has been overridden, we will
> check the syndrome for guest sync exception with an incorrect ESR_EL2
> value. So we want to save ESR_EL2 to cpu_user_regs as soon as the
> exception takes place in EL2 to avoid using an incorrect syndrome value.
>
> In order to save ESR_EL2, we added a 32-bit member hsr to cpu_user_regs.
> But while saving registers in trap entry, we use stp to save ELR and
> CPSR at the same time through 64-bit general registers. If we keep this
> code, the hsr will be overridden by upper 32-bit of CPSR. So adjust the
> code to use str to save ELR in a separate instruction and use stp to
> save CPSR and HSR at the same time through 32-bit general registers.
> This change affects the registers restore in trap exit, we can't use the
> ldp to restore ELR and CPSR from stack at the same time. We have to use
> ldr to restore them separately.
>
> Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
Reviewed-by: Julien Grall <julien.grall@xxxxxxx>

Cheers,

--
Julien Grall

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