Re: [PATCH] Remove support for ThumbEE

[Julien Grall <julien@xxxxxxx>]

Hi Michal,

On 13/04/2021 09:24, Michal Orzel wrote:
> ThumbEE(T32EE) was introduced in ARMv7 and removed in ARMv8.
> In 2011 ARM deprecated any use of the ThumbEE instruction set.
This doesn't mean this is not present in any CPU. In fact, in the same 
section (see A2.10 in ARM DDI 0406C.d):
"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.
  - Optional in implementations of the ARMv7-R profile.
"

> This feature is untested and as per my understanding
> there are no reported users for it. >
> Remove all the bits related to it.
>
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx> > ---
>   xen/arch/arm/cpufeature.c        |  3 +++
>   xen/arch/arm/domain.c            | 12 ------------
>   xen/arch/arm/setup.c             |  3 +--
>   xen/include/asm-arm/cpregs.h     |  6 ------
>   xen/include/asm-arm/cpufeature.h |  1 -
>   xen/include/asm-arm/domain.h     |  1 -
>   6 files changed, 4 insertions(+), 22 deletions(-)
>
> diff --git a/xen/arch/arm/cpufeature.c b/xen/arch/arm/cpufeature.c
> index 1d88783809..82265a72f4 100644
> --- a/xen/arch/arm/cpufeature.c
> +++ b/xen/arch/arm/cpufeature.c
> @@ -209,6 +209,9 @@ static int __init create_guest_cpuinfo(void)
>       guest_cpuinfo.pfr32.ras = 0;
>       guest_cpuinfo.pfr32.ras_frac = 0;
>   
> +    /* Hide ThumbEE support */
> +    guest_cpuinfo.pfr32.thumbee = 0;
Even if you hide the feature from the guest, the registers are still 
accessible. So you are not removing support but just opening a potential 
security hole as the registers now gets shared...
Looking at the spec, it doesn't look like it is possible to trap them.

In any case, the number of registers to save/restore is pretty limited. 
So I don't see the problem to keep the code around. It doesn't mean the 
feature is working, it just means we properly keep the domain isolated 
from each other.
Cheers,

--
Julien Grall