Re: [PATCH] accel/tcg: Introduce and use MO_ALIGN_TLB_ONLY

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 22/10/25 02:17, Richard Henderson wrote:
> For Arm, we need 3 cases: (1) the alignment required when accessing
> Normal memory, (2) the alignment required when accessing Device memory,
> and (3) the atomicity of the access.
> 
> When we added TLB_CHECK_ALIGNED, we assumed that cases 2 and 3 were
> identical, and thus used memop_atomicity_bits for TLB_CHECK_ALIGNED.
> 
> This is incorrect for multiple reasons, including that the atomicity
> of the access is adjusted depending on whether or not we are executing
> within a serial context.
> 
> For Arm, what is true is that there is an underlying alignment
> requirement of the access, and for that access Normal memory
> will support unalignement.
> 
> Introduce MO_ALIGN_TLB_ONLY to indicate that the alignment
> specified in MO_AMASK only applies when the TLB entry has
> TLB_CHECK_ALIGNED set; otherwise no alignment required.
> 
> Introduce memop_tlb_alignment_bits with an additional bool
> argument that specifies whether TLB_CHECK_ALIGNED is set.
> All other usage of memop_alignment_bits assumes it is not.
> 
> Remove memop_atomicity_bits as unused; it didn't properly
> support MO_ATOM_SUBWORD anyway.
> 
> Update target/arm finalize_memop_atom to set MO_ALIGN_TLB_ONLY
> when strict alignment isn't otherwise required.
> 
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3171
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
> 
> PS: There are a number of uses of align_mem for AdvSIMD, SVE and SME.
> I have not re-familiarized myself with the effects of SCR.A and
> Normal/Device memory for those cases.  I may well have missed something.
> 
> ---
>   include/exec/memop.h            | 43 +++++++++++++++------------------
>   target/arm/tcg/translate.h      |  4 +--
>   accel/tcg/cputlb.c              | 13 +---------
>   target/arm/ptw.c                |  2 +-
>   target/arm/tcg/translate-a64.c  | 10 +++-----
>   target/arm/tcg/translate-neon.c |  2 +-
>   tcg/tcg.c                       | 10 +++++---
>   7 files changed, 35 insertions(+), 49 deletions(-)

To the best of my knowledge,
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>