Re: [PATCH v3] xen/arm64: Zero the top 32 bits of gp registers on entry...

[Julien Grall <julien@xxxxxxx>]

Hi,

On 17/12/2021 07:21, Michal Orzel wrote:
> to hypervisor when switching from AArch32 state.
>
> According to section D1.20.2 of Arm Arm(DDI 0487A.j):
> "If the general-purpose register was accessible from AArch32 state the
> upper 32 bits either become zero, or hold the value that the same
> architectural register held before any AArch32 execution.
> The choice between these two options is IMPLEMENTATION DEFINED"
>
> Currently Xen does not ensure that the top 32 bits are zeroed and this
> needs to be fixed. The reason why is that there are places in Xen
> where we assume that top 32bits are zero for AArch32 guests.
> If they are not, this can lead to misinterpretation of Xen regarding
> what the guest requested. For example hypercalls returning an error
> encoded in a signed long like do_sched_op, do_hmv_op, do_memory_op
> would return -ENOSYS if the command passed as the first argument was
> clobbered.
>
> Create a macro clobber_gp_top_halves to clobber top 32 bits of gp
> registers when hyp == 0 (guest mode) and compat == 1 (AArch32 mode).
> Add a compile time check to ensure that save_x0_x1 == 1 if
> compat == 1.
>
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> ---
> Changes since v2:
> -add clobbering of w30
> Changes since v1:
> -put new code into macro
> -add compile time check for save_x0_x1
> ---
>   xen/arch/arm/arm64/entry.S | 29 +++++++++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
>
> diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
> index fc3811ad0a..e351ef8639 100644
> --- a/xen/arch/arm/arm64/entry.S
> +++ b/xen/arch/arm/arm64/entry.S
> @@ -102,6 +102,30 @@
>           .endif
>   
>           .endm
> +
> +/*
> + * Clobber top 32 bits of gp registers when switching from AArch32
> + */
> +        .macro clobber_gp_top_halves, compat, save_x0_x1
> +
> +        .if \compat == 1      /* AArch32 mode */
> +
> +        /*
> +         * save_x0_x1 is equal to 0 only for guest_sync (compat == 0).
> +         * Add a compile time check to avoid violating this rule.
> +         */
We may want in the future to allow save_x0_x1 == 1 with compat == 1 if 
we need to create fastpath like we did when entering AArch64.
So I would reword the comment to make clear this is an implementation 
decision. How about:
"At the moment, no-one is using save_x0_x1 == 0 with compat == 1. So the 
code is not handling it to simplify the implementation."
If you are happy with the new comment, I can update it on commit:

Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall