Re: [PATCH v3 1/2] powerpc: sstep: Fix load-store and update emulation

[Sandipan Das <sandipan@linux.ibm.com>]

On 04/02/21 12:44 pm, Sandipan Das wrote:
> The Power ISA says that the fixed-point load and update
> instructions must neither use R0 for the base address (RA)
> nor have the destination (RT) and the base address (RA) as
> the same register. Similarly, for fixed-point stores and
> floating-point loads and stores, the instruction is invalid
> when R0 is used as the base address (RA).
> 
> This is applicable to the following instructions.
>   * Load Byte and Zero with Update (lbzu)
>   * Load Byte and Zero with Update Indexed (lbzux)
>   * Load Halfword and Zero with Update (lhzu)
>   * Load Halfword and Zero with Update Indexed (lhzux)
>   * Load Halfword Algebraic with Update (lhau)
>   * Load Halfword Algebraic with Update Indexed (lhaux)
>   * Load Word and Zero with Update (lwzu)
>   * Load Word and Zero with Update Indexed (lwzux)
>   * Load Word Algebraic with Update Indexed (lwaux)
>   * Load Doubleword with Update (ldu)
>   * Load Doubleword with Update Indexed (ldux)
>   * Load Floating Single with Update (lfsu)
>   * Load Floating Single with Update Indexed (lfsux)
>   * Load Floating Double with Update (lfdu)
>   * Load Floating Double with Update Indexed (lfdux)
>   * Store Byte with Update (stbu)
>   * Store Byte with Update Indexed (stbux)
>   * Store Halfword with Update (sthu)
>   * Store Halfword with Update Indexed (sthux)
>   * Store Word with Update (stwu)
>   * Store Word with Update Indexed (stwux)
>   * Store Doubleword with Update (stdu)
>   * Store Doubleword with Update Indexed (stdux)
>   * Store Floating Single with Update (stfsu)
>   * Store Floating Single with Update Indexed (stfsux)
>   * Store Floating Double with Update (stfdu)
>   * Store Floating Double with Update Indexed (stfdux)
> 
> E.g. the following behaviour is observed for an invalid
> load and update instruction having RA = RT.
> 
> While an userspace program having an instruction word like
> 0xe9ce0001, i.e. ldu r14, 0(r14), runs without getting
> receiving a SIGILL on a Power system (observed on P8 and
> P9), the outcome of executing that instruction word varies
> and its behaviour can be considered to be undefined.
> 
> Attaching an uprobe at that instruction's address results
> in emulation which currently performs the load as well as
> writes the effective address back to the base register.
> This might not match the outcome from hardware.
> 
> To remove any inconsistencies, this adds additional checks
> for the aforementioned instructions to make sure that the
> emulation infrastructure treats them as unknown. The kernel
> can then fallback to executing such instructions on hardware.
> 
> Fixes: 0016a4cf5582 ("powerpc: Emulate most Book I instructions in emulate_step()")
> Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
> ---
> Previous versions can be found at:
> v2: https://lore.kernel.org/linuxppc-dev/20210203063841.431063-1-sandipan@linux.ibm.com/
> v1: https://lore.kernel.org/linuxppc-dev/20201119054139.244083-1-sandipan@linux.ibm.com/
> 
> Changes in v3:
> - Dropped CONFIG_PPC_FPU check as suggested by Michael.
> - Consolidated the checks as suggested by Naveen.

Sorry. Missed these in the changelog:
- Consolidated load/store changes into a single patch.
- Included floating-point load/store and update instructions.


- Sandipan