Re: [PATCH] ARM: Drop ThumbEE support

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Andrew and Julien,

R52 which i think is one of the few Arm CPUs that some people are looking at 
using
32bit instructions support Thumb-2 but not ThumbEE.

ThumbEE has been deprecated so I think Andrews changes (including the one
that traps any ThumbEE usage which is definitely required) are Ok.

Even on Cortex A15, I am not that sure that many people actually used ThumbEE.

Regards
Bertrand

> On 3 Dec 2025, at 22:02, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi Andrew,
> 
> On 03/12/2025 18:49, Andrew Cooper wrote:
>> On 03/12/2025 6:23 pm, Julien Grall wrote:
>>> Hi Andrew,
>>> 
>>> On 03/12/2025 17:16, Andrew Cooper wrote:
>>>> to keep it working, but there was apparently no use of ThumbEE
>>>> outside of demo
>>>> code, so simply drop it.
>>> 
>>> I am in favor of dropping support for ThumbEE for guest. But I am not
>>> sure I understand this comment.
>> There's no production use of ThumbEE known to ARM.
> 
> Interesting. The Arm Arm is less assertive and in fact confusing, the latest 
> spec released in 2018 (ARM DDI 0406C.d) has the following:
> 
> "
> From the publication of issue C.a of this manual, ARM deprecates any use of 
> the ThumbEE instruction set.
> 
> [...]
> 
> ThumbEE is both the name of the instruction set and the name of the extension 
> that provides support for that
> instruction set. The ThumbEE Extension is:
> • Required in implementations of the ARMv7-A profile.
> "
> 
> I have also checked the TRM of the Cortex-A15 (one of the Armv7 CPU we used 
> to have in the CI before gitlab) and it supports ThumbEE. So I think we need 
> to additional diff you sent in this patch.
> 
>>> Are you saying there are no processors supporting ThumbEE where Xen
>>> would run? Asking because below, you are removing code to context
>>> switch the ThumbEE registers. But I don't see any code that would
>>> ensure the registers are trapping (i.e. HSTR.TEE is set). So wouldn't
>>> this result to a cross-VM leak on those processors?
>>> 
>>> If we really don't want to support CPU where ThumbEE is available,
>>> then we should check that "cpu_has_thumbee" is 0.
>> The registers exist in ARM v7 only.  They do not exist in ARM v8.
>> I suppose yes this change would result in a cross-VM leak on hardware
>> supporting ThumbEE.
>> Can HSTR.TTEE be set unilaterally, or does it need gating on
>> cpu_has_thumbee?
> 
> From my understanding of the Armv7 specification, the bit doesn't need to be 
> gated. The Armv8 specification suggest it is RES0 so in theory it could be 
> used in the future. So the diff you sent is correct.
> 
>> Is setting HSTR.TTEE sufficient to cause an undefined instruction
>> exception to be thrown back at a guest which goes poking?  (I guess this
>> is really "will the default do the right thing")
> 
> I believe so. The default behavior in Xen is to inject an undefined exception 
> if it can't handle a trap.
> 
>> I'll freely admit that I'm out of my depth here, but the build failure
>> does need fixing.
> 
> I agree. Do we need to include the fix in 4.19 (I think Jan asked to cut it 
> tomorrow)? Thanks for sending the patch!
> 
> Cheers,
> 
> -- 
> Julien Grall