Re: [Qemu-arm] [Qemu-devel] [PATCH 08/13] target/arm: Add v8M stack checks for LDRD/STRD (imm)

[Richard Henderson]

On 10/2/18 11:35 AM, Peter Maydell wrote:
> Add the v8M stack checks for:
>  * LDRD (immediate)
>  * STRD (immediate)
> 
> Loads and stores are more complicated than ADD/SUB/MOV, because we
> must ensure that memory accesses below the stack limit are not
> performed, so we can't simply do the check when we actually update
> SP.
> 
> For these instructions, if the stack limit check triggers
> we must not:
>  * perform any memory access below the SP limit
>  * update PC, SP or the load/store base register
> but it is IMPDEF whether we:
>  * perform any accesses above or equal to the SP limit
>  * update destination registers for loads
> 
> For QEMU we choose to always check the limit before doing any other
> part of the load or store, so we won't update any registers or
> perform any memory accesses.
> 
> It is UNKNOWN whether the limit check triggers for a load or store
> where the initial SP value is below the limit and one of the stores
> would be below the limit, but the writeback moves SP to above the
> limit.  For QEMU we choose to trigger the check in this situation.
> 
> Note that limit checks happen only for loads and stores which update
> SP via writeback; they do not happen for loads and stores which
> simply use SP as a base register.
> 
> Signed-off-by: Peter Maydell <address@hidden>
> ---
>  target/arm/translate.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)

Reviewed-by: Richard Henderson <address@hidden>

r~