Re: [PATCH] iommu/vtd: fix address translation for superpages

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 10, 2023 at 12:00:51PM +0200, Jan Beulich wrote:
> On 10.05.2023 10:27, Roger Pau Monné wrote:
> > On Tue, May 09, 2023 at 06:06:45PM +0200, Jan Beulich wrote:
> >> On 09.05.2023 12:41, Roger Pau Monne wrote:
> >>> When translating an address that falls inside of a superpage in the
> >>> IOMMU page tables the fetching of the PTE physical address field
> >>> wasn't using dma_pte_addr(), which caused the returned data to be
> >>> corrupt as it would contain bits not related to the address field.
> >>
> >> I'm afraid I don't understand:
> >>
> >>> --- a/xen/drivers/passthrough/vtd/iommu.c
> >>> +++ b/xen/drivers/passthrough/vtd/iommu.c
> >>> @@ -359,16 +359,18 @@ static uint64_t addr_to_dma_page_maddr(struct 
> >>> domain *domain, daddr_t addr,
> >>>  
> >>>              if ( !alloc )
> >>>              {
> >>> -                pte_maddr = 0;
> >>>                  if ( !dma_pte_present(*pte) )
> >>> +                {
> >>> +                    pte_maddr = 0;
> >>>                      break;
> >>> +                }
> >>>  
> >>>                  /*
> >>>                   * When the leaf entry was requested, pass back the full 
> >>> PTE,
> >>>                   * with the address adjusted to account for the residual 
> >>> of
> >>>                   * the walk.
> >>>                   */
> >>> -                pte_maddr = pte->val +
> >>> +                pte_maddr +=
> >>>                      (addr & ((1UL << level_to_offset_bits(level)) - 1) &
> >>>                       PAGE_MASK);
> >>
> >> With this change you're now violating what the comment says (plus what
> >> the comment ahead of the function says). And it says what it says for
> >> a reason - see intel_iommu_lookup_page(), which I think your change is
> >> breaking.
> > 
> > Hm, but the code in intel_iommu_lookup_page() is now wrong as it takes
> > the bits in DMA_PTE_CONTIG_MASK as part of the physical address when
> > doing the conversion to mfn?  maddr_to_mfn() doesn't perform a any
> > masking to remove the bits above PADDR_BITS.
> 
> Oh, right. But that's a missing dma_pte_addr() in intel_iommu_lookup_page()
> then. (It would likely be better anyway to switch "uint64_t val" to
> "struct dma_pte pte" there, to make more visible that it's a PTE we're
> dealing with.) I indeed overlooked this aspect when doing the earlier
> change.

I guess I'm still confused, as the other return value for target == 0
(when the address is not part of a superpage) does return
dma_pte_addr(pte).  I think that needs further fixing then.

Thanks, Roger.