Re: [PATCH v4 09/19] hw/arm/smmu-common: Rework TLB lookup for nesting

[Mostafa Saleh <smostafa@google.com>]

Hi Jean,

On Thu, Jul 04, 2024 at 07:12:35PM +0100, Jean-Philippe Brucker wrote:
> On Mon, Jul 01, 2024 at 11:02:31AM +0000, Mostafa Saleh wrote:
> > In the next patch, combine_tlb() will be added which combines 2 TLB
> > entries into one for nested translations, which chooses the granule
> > and level from the smallest entry.
> > 
> > This means that with nested translation, an entry can be cached with
> > the granule of stage-2 and not stage-1.
> > 
> > However, currently, the lookup for an IOVA is done with input stage
> > granule, which is stage-1 for nested configuration, which will not
> > work with the above logic.
> > This patch reworks lookup in that case, so it falls back to stage-2
> > granule if no entry is found using stage-1 granule.
> 
> Why not initialize tt_combined to the minimum granule of stages 1 and 2?
> It looks like you introduced it for this. I'm wondering if we lookup the
> wrong IOVA if changing the granule size after the address is masked in
> smmu_translate()

I am not sure I fully understand, but I don’t think that would work as it is
not guaranteed that the minimum granule is the one that would be cached,
as we might hit block mappings.

The IOVA at first is masked with the first stage mask for the expected page
address, and the lookup logic would mask the address for each level look up,
so It should match the alignment of the cached page of that granule and level,
and as the combine logic is done with the aligned_addr it is guaranteed by
construction that it has to be aligned with stage-1.

Thanks,
Mostafa

> 
> Thanks,
> Jean
> 
> > 
> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
> > ---
> >  hw/arm/smmu-common.c | 36 ++++++++++++++++++++++++++++++++++--
> >  1 file changed, 34 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/arm/smmu-common.c b/hw/arm/smmu-common.c
> > index 21982621c0..0840b5cffd 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 +90,36 @@ SMMUTLBEntry *smmu_iotlb_lookup(SMMUState *bs, SMMUTransCfg *cfg,
> >          }
> >          level++;
> >      }
> > +    return entry;
> > +}
> > +
> > +/**
> > + * smmu_iotlb_lookup - Look up for a TLB entry.
> > + * @bs: SMMU state which includes the TLB instance
> > + * @cfg: Configuration of the translation
> > + * @tt: Translation table info (granule and tsz)
> > + * @iova: IOVA address to lookup
> > + *
> > + * returns a valid entry on success, otherwise NULL.
> > + * In case of nested translation, tt can be updated to include
> > + * the granule of the found entry as it might different from
> > + * the IOVA granule.
> > + */
> > +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;
> > +        entry = smmu_iotlb_lookup_all_levels(bs, cfg, tt, iova);
> > +    }
> >  
> >      if (entry) {
> >          cfg->iotlb_hits++;
> > -- 
> > 2.45.2.803.g4e1b14247a-goog
> >