Re: [PATCH 3/7] target/arm: Don't NOCP fault for FPCXT_NS accesses

[Richard Henderson <richard.henderson@linaro.org>]

On 6/18/21 7:10 AM, Peter Maydell wrote:
> The M-profile architecture requires that accesses to FPCXT_NS when
> there is no active FP state must not take a NOCP fault even if the
> FPU is disabled. We were not implementing this correctly, because
> in our decode we catch the NOCP faults early in m-nocp.decode.
> 
> Fix this bug by moving all the handling of M-profile FP system
> register accesses from vfp.decode into m-nocp.decode and putting
> it above the NOCP blocks. This provides the correct behaviour:
>   * for accesses other than FPCXT_NS the trans functions call
>     vfp_access_check(), which will check for FPU disabled and
>     raise a NOCP exception if necessary
>   * for FPCXT_NS we have the special case code that doesn't
>     call vfp_access_check()
>   * when these trans functions want to raise an UNDEF they return
>     false, so the decoder will fall through into the NOCP blocks.
>     This means that NOCP correctly takes precedence over UNDEF
>     for these insns. (This is a difference from the other insns
>     handled by m-nocp.decode, where UNDEF takes precedence and
>     which we implement by having those trans functions call
>     unallocated_encoding() in the appropriate places.)
> 
> [Note for backport to stable: this commit has a semantic dependency
> on commit 9a486856e9173af, which was not marked as cc-stable because
> we didn't know we'd need it for a for-stable bugfix.]
> 
> Cc:qemu-stable@nongnu.org
> Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> ---
> A big patch, but it's almost entirely code movement.
> ---

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~