[RFC PATCH 0/1] xen/arm: smccc: preserve arguments before register setup

[Jan Setje-Eilers <Jan.SetjeEilers@xxxxxxxxxx>]

Hi Andrew,

While investigating an SMC forwarding failure after updating to Xen 4.22,
I found that firmware could receive different values from those passed to
arm_smccc_1_1_smc().

Several public Xen callers pass get_user_reg() calls directly to this
helper.  This includes the SCMI, ZynqMP EEMI, and i.MX forwarding paths.
The helper puts each value into its SMC argument register as the
expression is evaluated.  A later get_user_reg() call can then overwrite a
register prepared for an earlier argument.

The change in generated code appears after:

67bcf5eae709 ("xen/arm: Simplify type handling for SMCCC declarations")

I am not sure which behavior the helper is intended to provide. Should
arm_smccc_1_1_smc() continue to accept arguments such as get_user_reg()
calls, or should callers save those values in local variables first?

The attached patch is a candidate for the first option. It restores
temporary variables inside the helper, allowing all calls to finish before
x0-x7 are prepared for the SMC instruction. I am not able to validate the
public SCMI and ZynqMP paths on hardware, but their generated code now
finishes every get_user_reg() call before the final register setup and
smc. With this change, SMCs also work as intended on the device I'm
using.

If the intent is not to accept function calls as arguments, we should
probably fix the SCMI, ZynqMP EEMI, and i.MX callers.

Given the existing use cases, this seems like a bug, but your patch looks
intentional. It's very possible I'm missing the intended design. I would
appreciate your view on the intended contract.

Thanks for any advice or help with this.

Jan Setje-Eilers (1):
  xen/arm: smccc: preserve arguments before register setup

 xen/arch/arm/include/asm/smccc.h | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

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2.47.3