Re: [PATCH v1 1/1] target/arm: Drop access_el3_aa32ns()

[Peter Maydell]

On Tue, 28 Apr 2020 at 17:03, Edgar E. Iglesias
<address@hidden> wrote:
>
> From: "Edgar E. Iglesias" <address@hidden>
>
> Calling access_el3_aa32ns() works for AArch32 only cores
> but it does not handle 32-bit EL2 on top of 64-bit EL3
> for mixed 32/64-bit cores.
>
> Fold access_el3_aa32ns() into access_el3_aa32ns_aa64any()
> and replace all direct uses of the aa32 only version with
> access_el3_aa32ns_aa64any().
>
> Fixes: 68e9c2fe65 ("target-arm: Add VTCR_EL2")
> Reported-by: Laurent Desnogues <address@hidden>
> Signed-off-by: Edgar E. Iglesias <address@hidden>

So, this is definitely a bug, but I think we could be
clearer about what we're fixing.

For all these registers, the way the Arm ARM pseudocode phrases
this access check is:
 * for the AArch64 view of the register, no check
 * for the AArch32 view of the register:
      ...
   elsif PSTATE.EL == EL2 then
      return VTTBR;
   elsif PSTATE.EL == EL3 then
      if SCR.NS == '0' then
          UNDEFINED;
      else
          return VTTBR;
(similarly for the write path). We don't implement the HSTR.T2
traps, so for us these registers are all .access = PL2_RW and
we just UNDEF for all EL0/EL1 accesses.

So what we're really trying to check for is "current EL is EL3
and we are AArch32 and SCR.NS == '0'". Because it's not possible
to be in AArch32 Hyp with SCR.NS == 0, the check we make in
your function is an equivalent test, but we could improve
the comments:
> ---
>  target/arm/helper.c | 34 ++++++++++------------------------
>  1 file changed, 10 insertions(+), 24 deletions(-)
>
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index 7e9ea5d20f..888f5f2314 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -504,29 +504,15 @@ void init_cpreg_list(ARMCPU *cpu)
>  /*
>   * Some registers are not accessible if EL3.NS=0 and EL3 is using AArch32 but
>   * they are accessible when EL3 is using AArch64 regardless of EL3.NS.

This could be rewritten as:
   Some registers are not accessible from AArch32 EL3 if SCR.NS == 0.

> - *
> - * access_el3_aa32ns: Used to check AArch32 register views.
> - * access_el3_aa32ns_aa64any: Used to check both AArch32/64 register views.
>   */
> -static CPAccessResult access_el3_aa32ns(CPUARMState *env,
> -                                        const ARMCPRegInfo *ri,
> -                                        bool isread)
> -{
> -    bool secure = arm_is_secure_below_el3(env);
> -
> -    assert(!arm_el_is_aa64(env, 3));
> -    if (secure) {
> -        return CP_ACCESS_TRAP_UNCATEGORIZED;
> -    }
> -    return CP_ACCESS_OK;
> -}
> -
>  static CPAccessResult access_el3_aa32ns_aa64any(CPUARMState *env,
>                                                  const ARMCPRegInfo *ri,
>                                                  bool isread)
>  {
> -    if (!arm_el_is_aa64(env, 3)) {
> -        return access_el3_aa32ns(env, ri, isread);
> +    bool secure = arm_is_secure_below_el3(env);
> +
> +    if (!arm_el_is_aa64(env, 3) && secure) {

We could either rephrase this as
       if (!is_a64(env) && arm_current_el(env) == 3 &&
           arm_is_secure_below_el3(env)) {

or just have a comment
       /*
        * This access function is only used with .access = PL2_RW
        * registers, so we are in AArch32 EL3 with SCR.NS == 0
        * if and only if EL3 is AArch32 and SCR.NS == 0, because
        * if SCR.NS == 0 we cannot be in EL2.
        */

depending on how much you proritize a more efficient test
over a more clearly correct test :-)

> +        return CP_ACCESS_TRAP_UNCATEGORIZED;
>      }
>      return CP_ACCESS_OK;
>  }

Also, once we don't have a distinction between two different
flavours of this access function we should use the simpler
"access_el2_aa32ns", rather than ending up using the longer
name for the one version of the function we're keeping.

thanks
-- PMM