Re: [Qemu-devel] [PATCH v2 3/3] target-arm: Implement the S2 MMU inputsize > pamax check

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Fri, Jan 22, 2016 at 10:28:43AM +0000, Alex Bennée wrote:
> 
> Edgar E. Iglesias <edgar.iglesias@gmail.com> writes:
> 
> > From: "Edgar E. Iglesias" <edgar.iglesias@xilinx.com>
> >
> > Implement the inputsize > pamax check for Stage 2 translations.
> > We have multiple choices for how to respond to errors and
> > choose to fault.
> >
> > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
> > ---
> >  target-arm/helper.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/target-arm/helper.c b/target-arm/helper.c
> > index 4abeb4d..9a7ff5e 100644
> > --- a/target-arm/helper.c
> > +++ b/target-arm/helper.c
> > @@ -6808,7 +6808,7 @@ static bool get_phys_addr_lpae(CPUARMState *env, target_ulong address,
> >           */
> >          int startlevel = extract32(tcr->raw_tcr, 6, 2);
> >          unsigned int pamax = arm_pamax(cpu);
> > -        bool ok;
> > +        bool ok = true;
> >
> >          if (va_size == 32 || stride == 9) {
> >              /* AArch32 or 4KB pages */
> > @@ -6818,9 +6818,17 @@ static bool get_phys_addr_lpae(CPUARMState *env, target_ulong address,
> >              level = 3 - startlevel;
> >          }
> >
> > -        /* Check that the starting level is valid. */
> > -        ok = check_s2_startlevel(cpu, va_size == 64, level,
> > -                                 inputsize, stride, pamax);
> > +        if (va_size == 64 &&
> > +            inputsize > pamax &&
> > +            (arm_el_is_aa64(env, 1) || inputsize > 40)) {
> 
> If va_size == 64 doesn't that imply arm_el_is_aa64(env, 1)? Looking
> further up the function it seems that is what sets va_size in the first
> place. I think that makes the inputsize > 40 check redundant.

va_size == 64 is true if the EL corresponding to the translation _regime_
is in 64 bit mode (in this case EL2).

EL1 may very well be in 32bit mode.


> 
> > +            /* We have multiple choices but choose to fault.  */
> > +            ok = false;
> > +        }
> > +        if (ok) {
> > +            /* Check that the starting level is valid. */
> > +            ok = check_s2_startlevel(cpu, va_size == 64, level,
> > +                                     inputsize, stride, pamax);
> > +        }
> >          if (!ok) {
> >              /* AArch64 reports these as level 0 faults.
> >               * AArch32 reports these as level 1 faults.
> 
> I'm not a fan of the ok = true / ok = false / ok =
> check_s2_start_level() / if (!ok) ping-pong here as it is hard to
> follow. I'm not sure how you could make it cleaner to follow though.
> Maybe something like:
> 
>         /* For stage 2 translations the starting level is specified by the
>          * VTCR_EL2.SL0 field (whose interpretation depends on the page size)
>          */
>         int startlevel = extract32(tcr->raw_tcr, 6, 2);
>         unsigned int pamax = arm_pamax(cpu);
>         bool is_aarch64_regime = (va_size == 64);
>         bool ok;
> 
>         if (va_size == 32 || stride == 9) {
>             /* AArch32 or 4KB pages */
>             level = 2 - startlevel;
>         } else {
>             /* 16KB or 64KB pages */
>             level = 3 - startlevel;
>         }
> 
>         if (is_aarch64_regime &&
>             inputsize > pamax) {
>             /* We have multiple choices but choose to fault.  */
>             ok = false;
>         } else {
>             /* Check that the starting level is valid. */
>             ok = check_s2_startlevel(cpu, is_aarch64_regime, level,
>                                      inputsize, stride, pamax);
>         }
>         if (!ok) {
>             /* AArch64 reports these as level 0 faults.
>              * AArch32 reports these as level 1 faults.
>              */
>             level = is_aarch64_regime ? 0 : 1;
>             fault_type = translation_fault;
>             goto do_fault;
>         }
> 
> But I'm wondering if it just makes more sense to push the:
> 
>     is_aarch64_regime && inputsize > pamax
> 
> Check into check_s2_startlevel? Then you could just have a simple call
> which succeeds or falls through to a fault?

Yeah, I guess we could rename check_s2_startlevel to something more generic
and move all the checks there. I don't feel very strongly about either way...

Thanks,
Edgar



> 
>         /* Check that the starting level is valid. */
>         if (!check_s2_startlevel(cpu, is_aarch64_regime, level,
>                                      inputsize, stride, pamax) ){
>             /* AArch64 reports these as level 0 faults.
>              * AArch32 reports these as level 1 faults.
>              */
>             level = is_aarch64_regime ? 0 : 1;
>             fault_type = translation_fault;
>             goto do_fault;
>         }
> 
> --
> Alex Bennée