Re: [RFC PATCH v3 09/18] hw/arm/smmu-common: Rework TLB lookup for nesting

[Mostafa Saleh <smostafa@google.com>]

Hi Eric,

On Mon, May 20, 2024 at 10:27:50AM +0200, Eric Auger wrote:
> Hi Mostafa,
> 
> On 4/29/24 05:23, Mostafa Saleh wrote:
> > In the previous patch, comine_tlb() was added which combines 2 TLB
> > entries into one, which chooses the granule and level from the
> > smallest entry.
> >
> > This means that a nested translation, an entry can be cached with the
> > granule of stage-2 and not stage-1.
> >
> > However, the lookup for an IOVA in nested configuration is done with
> > stage-1 granule, this patch reworks lookup in that case, so it falls
> > back to stage-2 granule if no entry is found using stage-1 granule.
> >
> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
> > ---
> >  hw/arm/smmu-common.c | 24 ++++++++++++++++++++++--
> >  1 file changed, 22 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/arm/smmu-common.c b/hw/arm/smmu-common.c
> > index 0d6945fa54..c67af3bc6d 100644
> > --- a/hw/arm/smmu-common.c
> > +++ b/hw/arm/smmu-common.c
> > @@ -66,8 +66,10 @@ SMMUIOTLBKey smmu_get_iotlb_key(int asid, int vmid, uint64_t iova,
> >      return key;
> >  }
> >  
> > -SMMUTLBEntry *smmu_iotlb_lookup(SMMUState *bs, SMMUTransCfg *cfg,
> > -                                SMMUTransTableInfo *tt, hwaddr iova)
> > +static SMMUTLBEntry *smmu_iotlb_lookup_all_levels(SMMUState *bs,
> > +                                                  SMMUTransCfg *cfg,
> > +                                                  SMMUTransTableInfo *tt,
> > +                                                  hwaddr iova)
> >  {
> >      uint8_t tg = (tt->granule_sz - 10) / 2;
> >      uint8_t inputsize = 64 - tt->tsz;
> > @@ -88,6 +90,24 @@ SMMUTLBEntry *smmu_iotlb_lookup(SMMUState *bs, SMMUTransCfg *cfg,
> >          }
> >          level++;
> >      }
> > +    return entry;
> > +}
> > +
> > +SMMUTLBEntry *smmu_iotlb_lookup(SMMUState *bs, SMMUTransCfg *cfg,
> > +                                SMMUTransTableInfo *tt, hwaddr iova)
> > +{
> > +    SMMUTLBEntry *entry = NULL;
> > +
> > +    entry = smmu_iotlb_lookup_all_levels(bs, cfg, tt, iova);
> > +    /*
> > +     * For nested translation also try the s2 granule, as the TLB will insert
> > +     * it if the size of s2 tlb entry was smaller.
> > +     */
> > +    if (!entry && (cfg->stage == SMMU_NESTED) &&
> > +        (cfg->s2cfg.granule_sz != tt->granule_sz)) {
> > +        tt->granule_sz = cfg->s2cfg.granule_sz;
> is it safe to alter the tt->granule_sz without restoring it? In the
> positive I think this would deserve a comment.

It should be safe in the current usage, I will add a comment to
clarify how the function behaves (something as the the granule_sz
would be updated to the entry tg if found)

Thanks,
Mostafa
> 
> Eric
> > +        entry = smmu_iotlb_lookup_all_levels(bs, cfg, tt, iova);
> > +    }
> >  
> >      if (entry) {
> >          cfg->iotlb_hits++;
>