Re: [PATCH] target/arm: Permit T32 LDM with single register

[Alex Bennée]

Peter Maydell <peter.maydell@linaro.org> writes:

> For the Thumb T32 encoding of LDM, if only a single register is
> specified in the register list this instruction is UNPREDICTABLE,
> with the following choices:
>  * instruction UNDEFs
>  * instruction is a NOP
>  * instruction loads a single register
>  * instruction loads an unspecified set of registers
>
> Currently we choose to UNDEF (a behaviour chosen in commit
> 4b222545dbf30 in 2019; previously we treated it as "load the
> specified single register").
>
> Unfortunately there is real world code out there (which shipped in at
> least Android 11, 12 and 13) which incorrectly uses this
> UNPREDICTABLE insn on the assumption that it does a single register
> load, which is (presumably) what it happens to do on real hardware,
> and is also what it does on the equivalent A32 encoding.
>
> Revert to the pre-4b222545dbf30 behaviour of not UNDEFing
> for this T32 encoding.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1799
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro