Re: [PATCH 3/3] target/arm: Handle trapping to EL2 of AArch32 VMRS instructions

[Marc Zyngier <maz@kernel.org>]

Hi Peter,

Thanks for having a look at this.

On 2019-11-28 16:43, Peter Maydell wrote:
> On Thu, 28 Nov 2019 at 16:17, Marc Zyngier <maz@kernel.org> wrote:
>>
>> HCR_EL2.TID3 requires that AArch32 reads of MVFR[012] are trapped to
>> EL2, and that HCR_EL2.TID0 does the same for reads of FPSID.
>> In order to handle this, introduce a new TCG helper function that
>> checks for these control bits before executing the VMRC instruction.
>>
>> Tested with a hacked-up version of KVM/arm64 that sets the control
>> bits for 32bit guests.
>>
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>> ---
>>  target/arm/helper-a64.h        |  2 ++
>>  target/arm/internals.h         |  8 ++++++++
>>  target/arm/translate-vfp.inc.c | 12 +++++++++---
>>  target/arm/vfp_helper.c        | 27 +++++++++++++++++++++++++++
>>  4 files changed, 46 insertions(+), 3 deletions(-)
>>
>> diff --git a/target/arm/helper-a64.h b/target/arm/helper-a64.h
>> index a915c1247f..311ced44e6 100644
>> --- a/target/arm/helper-a64.h
>> +++ b/target/arm/helper-a64.h
>> @@ -102,3 +102,5 @@ DEF_HELPER_FLAGS_3(autda, TCG_CALL_NO_WG, i64, 
>> env, i64, i64)
>>  DEF_HELPER_FLAGS_3(autdb, TCG_CALL_NO_WG, i64, env, i64, i64)
>>  DEF_HELPER_FLAGS_2(xpaci, TCG_CALL_NO_RWG_SE, i64, env, i64)
>>  DEF_HELPER_FLAGS_2(xpacd, TCG_CALL_NO_RWG_SE, i64, env, i64)
>> +
>> +DEF_HELPER_3(check_hcr_el2_trap, void, env, int, int)
>> diff --git a/target/arm/internals.h b/target/arm/internals.h
>> index f5313dd3d4..5a55e960de 100644
>> --- a/target/arm/internals.h
>> +++ b/target/arm/internals.h
>> @@ -430,6 +430,14 @@ static inline uint32_t syn_simd_access_trap(int 
>> cv, int cond, bool is_16bit)
>>          | (cv << 24) | (cond << 20) | (1 << 5);
>>  }
>>
>> +static inline uint32_t syn_vmrs_trap(int rt, int reg)
>> +{
>> +    return (EC_FPIDTRAP << ARM_EL_EC_SHIFT)
>> +        | ARM_EL_IL
>> +        | (1 << 24) | (0xe << 20) | (7 << 14)
>> +        | (reg << 10) | (rt << 5) | 1;
>> +}
>> +
>>  static inline uint32_t syn_sve_access_trap(void)
>>  {
>>      return EC_SVEACCESSTRAP << ARM_EL_EC_SHIFT;
>> diff --git a/target/arm/translate-vfp.inc.c 
>> b/target/arm/translate-vfp.inc.c
>> index 85c5ef897b..4c435b6c35 100644
>> --- a/target/arm/translate-vfp.inc.c
>> +++ b/target/arm/translate-vfp.inc.c
>> @@ -759,15 +759,21 @@ static bool trans_VMSR_VMRS(DisasContext *s, 
>> arg_VMSR_VMRS *a)
>>      }
>>
>>      if (a->l) {
>> +        TCGv_i32 tcg_rt, tcg_reg;
>> +
>>          /* VMRS, move VFP special register to gp register */
>>          switch (a->reg) {
>> +        case ARM_VFP_MVFR0:
>> +        case ARM_VFP_MVFR1:
>> +        case ARM_VFP_MVFR2:
>>          case ARM_VFP_FPSID:
>> +            tcg_rt = tcg_const_i32(a->rt);
>> +            tcg_reg = tcg_const_i32(a->reg);
>
> Since the syndrome value depends only on these two things,
> you might as well generate the full syndrome value at
> translate time rather than doing it at runtime; then
> you only need to pass one thing through to the helper rather
> than two.

OK. This means that the register check in check_hcr_el2_trap
will need to extract the register value from the syndrome.
Not a big deal, but maybe slightly less readable.

>
>> +            gen_helper_check_hcr_el2_trap(cpu_env, tcg_rt, 
>> tcg_reg);
>
> This helper call is potentially going to throw an exception
> at runtime. QEMU's JIT doesn't write back all the state
> of the CPU to the CPU state structure fields for helper
> calls, so to avoid losing non-written-back state there are
> two possible approaches:
>
> (1) manually write back the state before the call; for
> aarch32 this looks like
>             gen_set_condexec(s);
>             gen_set_pc_im(s, s->pc_curr);
> (you can see this done before we call the access_check_cp_reg()
> helper, for instance)
>
> (2) in the helper function, instead of raise_exception(),
> call raise_exception_ra(..., GETPC())
> This says "when we take the exception, also re-sync the
> CPU state by looking at the host PC value in the JITted
> code (ie the address of the callsite of the helper) and
> looking through a table for this translation block that
> cross-references the host PC against the guest PC and
> condexec values for that point in execution".
>
> Option 1 is better if the expectation is that the trap will
> be taken always, often or usually; option 2 is what we
> use if the trap is unlikely (it's how we handle
> exceptions on guest load/store insns, which are the main
> reason we have the mechanism at all).
>
> Since it's unlikely that guest code will be doing ID
> register accesses in hot codepaths, I'd go with option 1,
> mostly just for consistency with how we do coprocessor
> register access-check function calls.

Ah, very interesting stuff. There is a lot of "magic" happening
in QEMU, and I wondered about the emulated state at some point,
until I forgot about it!

On a vaguely tangential subject, how are conditional instructions
JIT-ed? I could perfectly imagine a conditional VMRS instruction,
but none of the code I looked at seem to care about it. Or is
that done before the access itself is actually emitted?

>
>> +            /* fall through */
>>          case ARM_VFP_FPEXC:
>>          case ARM_VFP_FPINST:
>>          case ARM_VFP_FPINST2:
>> -        case ARM_VFP_MVFR0:
>> -        case ARM_VFP_MVFR1:
>> -        case ARM_VFP_MVFR2:
>>              tmp = load_cpu_field(vfp.xregs[a->reg]);
>>              break;
>>          case ARM_VFP_FPSCR:
>> diff --git a/target/arm/vfp_helper.c b/target/arm/vfp_helper.c
>> index 9710ef1c3e..44e538e51c 100644
>> --- a/target/arm/vfp_helper.c
>> +++ b/target/arm/vfp_helper.c
>> @@ -1322,4 +1322,31 @@ float64 HELPER(frint64_d)(float64 f, void 
>> *fpst)
>>      return frint_d(f, fpst, 64);
>>  }
>>
>> +void HELPER(check_hcr_el2_trap)(CPUARMState *env, int rt, int reg)
>> +{
>> +    if (arm_current_el(env) != 1) {
>> +        return;
>> +    }
>> +
>> +    switch (reg) {
>> +    case ARM_VFP_MVFR0:
>> +    case ARM_VFP_MVFR1:
>> +    case ARM_VFP_MVFR2:
>> +        if (!(arm_hcr_el2_eff(env) & HCR_TID3)) {
>> +            return;
>> +        }
>> +        break;
>> +    case ARM_VFP_FPSID:
>> +        if (!(arm_hcr_el2_eff(env) & HCR_TID0)) {
>> +            return;
>> +        }
>> +        break;
>> +    default:
>> +        /* Shouldn't be here... */
>> +        return;
>
> We usually write 'impossible' default cases as:
>            g_assert_not_reached();

Noted, thanks.

I'll wait a bit for additional reviews (if any), and then repost the
series with these fixes in.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...