Re: [PATCH v3 3/6] iommu: add ARM short descriptor page table allocator.

[Will Deacon]

On Fri, Jul 24, 2015 at 06:24:26AM +0100, Yong Wu wrote:
> On Tue, 2015-07-21 at 18:11 +0100, Will Deacon wrote:
> > On Thu, Jul 16, 2015 at 10:04:32AM +0100, Yong Wu wrote:
> > > +/* level 2 pagetable */
> > > +#define ARM_SHORT_PTE_TYPE_LARGE               BIT(0)
> > > +#define ARM_SHORT_PTE_SMALL_XN                 BIT(0)
> > > +#define ARM_SHORT_PTE_TYPE_SMALL               BIT(1)
> > > +#define ARM_SHORT_PTE_B                                BIT(2)
> > > +#define ARM_SHORT_PTE_C                                BIT(3)
> > > +#define ARM_SHORT_PTE_SMALL_TEX0               BIT(6)
> > > +#define ARM_SHORT_PTE_IMPLE                    BIT(9)
> >
> > This is AP[2] for small pages.
> 
> Sorry, In our pagetable bit9 in PGD and PTE is PA[32] that is for  the
> dram size over 4G. I didn't care it is different in PTE of the standard
> spec.
> And I don't use the AP[2] currently, so I only delete this line in next
> time.

Is this related to the "special bit". What would be good is a comment
next to the #define for the quirk describing *exactly* that differs in
your implementation. Without that, it's very difficult to know what is
intentional and what is actually broken.

> > > +static arm_short_iopte
> > > +__arm_short_pte_prot(struct arm_short_io_pgtable *data, int prot, bool large)
> > > +{
> > > +       arm_short_iopte pteprot;
> > > +
> > > +       pteprot = ARM_SHORT_PTE_S | ARM_SHORT_PTE_nG;
> > > +       pteprot |= large ? ARM_SHORT_PTE_TYPE_LARGE :
> > > +                               ARM_SHORT_PTE_TYPE_SMALL;
> > > +       if (prot & IOMMU_CACHE)
> > > +               pteprot |=  ARM_SHORT_PTE_B | ARM_SHORT_PTE_C;
> > > +       if (prot & IOMMU_WRITE)
> > > +               pteprot |= large ? ARM_SHORT_PTE_LARGE_TEX0 :
> > > +                               ARM_SHORT_PTE_SMALL_TEX0;
> >
> > This doesn't make any sense. TEX[2:0] is all about memory attributes, not
> > permissions, so you're making the mapping write-back, write-allocate but
> > that's not what the IOMMU_* values are about.
> 
>      I will delete it.

Well, can you not control mapping permissions with the AP bits? The idea
of the IOMMU flags are:

  IOMMU_CACHE : Install a normal, cacheable mapping (you've got this right)
  IOMMU_READ : Allow read access for the device
  IOMMU_WRITE : Allow write access for the device
  IOMMU_NOEXEC : Disallow execute access for the device

so the caller to iommu_map passes in a bitmap of these, which you need to
encode in the page-table entry.

> > > +static int
> > > +_arm_short_map(struct arm_short_io_pgtable *data,
> > > +              unsigned int iova, phys_addr_t paddr,
> > > +              arm_short_iopte pgdprot, arm_short_iopte pteprot,
> > > +              bool large)
> > > +{
> > > +       const struct iommu_gather_ops *tlb = data->iop.cfg.tlb;
> > > +       arm_short_iopte *pgd = data->pgd, *pte;
> > > +       void *cookie = data->iop.cookie, *pte_va;
> > > +       unsigned int ptenr = large ? 16 : 1;
> > > +       int i, quirk = data->iop.cfg.quirks;
> > > +       bool ptenew = false;
> > > +
> > > +       pgd += ARM_SHORT_PGD_IDX(iova);
> > > +
> > > +       if (!pteprot) { /* section or supersection */
> > > +               if (quirk & IO_PGTABLE_QUIRK_SHORT_MTK)
> > > +                       pgdprot &= ~ARM_SHORT_PGD_SECTION_XN;
> > > +               pte = pgd;
> > > +               pteprot = pgdprot;
> > > +       } else {        /* page or largepage */
> > > +               if (quirk & IO_PGTABLE_QUIRK_SHORT_MTK) {
> > > +                       if (large) { /* special Bit */
> >
> > This definitely needs a better comment! What exactly are you doing here
> > and what is that quirk all about?
> 
> I use this quirk is for MTK Special Bit as we don't have the XN bit in
> pagetable.

I'm still not really clear about what this is.

> > > +               if (!(*pgd)) {
> > > +                       pte_va = kmem_cache_zalloc(data->ptekmem, GFP_ATOMIC);
> > > +                       if (unlikely(!pte_va))
> > > +                               return -ENOMEM;
> > > +                       ptenew = true;
> > > +                       *pgd = virt_to_phys(pte_va) | pgdprot;
> > > +                       kmemleak_ignore(pte_va);
> > > +                       tlb->flush_pgtable(pgd, sizeof(*pgd), cookie);
> >
> > I think you need to flush this before it becomes visible to the walker.
> 
> I have flushed pgtable here, Do you meaning flush tlb here?

No. afaict, you allocate the pte table using kmem_cache_zalloc but you never
flush it. However, you update the pgd to point at this table, so the walker
can potentially see garbage instead of the zeroed entries.

Will
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