Re: [Xen-devel] [PATCH v2] x86/p2m-pt: tighten conditions of IOMMU mapping updates

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/10/15 11:25, Jan Beulich wrote:
> Whether the MFN changes does not depend on the new entry being valid
> (but solely on the old one), and the need to update or TLB-flush also
> depends on permission changes.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Split from larger patch. Fix logic determining whether to update/
>     flush IOMMU mappings.
> ---
> TBD: As already mentioned on the large-page-MMIO-mapping patch, there
>      is an apparent inconsistency with PoD handling: 2M mappings get
>      valid entries created, while 4k mappings don't. It would seem to
>      me that the 4k case needs changing, even if today this may only
>      be a latent bug. Question of course is why we don't rely on
>      p2m_type_to_flags() doing its job properly and instead special
>      case certain P2M types.
> TBD: Rip out hap-pt-share code from the file altogether?
>
> --- a/xen/arch/x86/mm/p2m-pt.c
> +++ b/xen/arch/x86/mm/p2m-pt.c
> @@ -494,7 +494,18 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>      l3_pgentry_t l3e_content;
>      int rc;
>      unsigned int iommu_pte_flags = p2m_get_iommu_flags(p2mt);
> -    unsigned long old_mfn = 0;
> +    /*
> +     * old_mfn and iommu_old_flags control possible flush/update needs on the
> +     * IOMMU: We need to flush when MFN or flags (i.e. permissions) change.
> +     * iommu_old_flags being initialized to zero covers the case of the entry
> +     * getting replaced being a non-present (leaf or intermediate) one. For
> +     * present leaf entries the real value will get calculated below, while
> +     * for present intermediate entries ~0 (guaranteed != iommu_pte_flags)
> +     * will be used (to cover all cases of what the leaf entries underneath
> +     * the intermediate one might be).
> +     */
> +    unsigned int flags, iommu_old_flags = 0;
> +    unsigned long old_mfn = INVALID_MFN;
>  
>      ASSERT(sve != 0);
>  
> @@ -543,12 +554,20 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>                                     L3_PAGETABLE_SHIFT - PAGE_SHIFT,
>                                     L3_PAGETABLE_ENTRIES);
>          ASSERT(p2m_entry);
> -        if ( (l1e_get_flags(*p2m_entry) & _PAGE_PRESENT) &&
> -             !(l1e_get_flags(*p2m_entry) & _PAGE_PSE) )
> +        flags = l1e_get_flags(*p2m_entry);
> +        if ( flags & _PAGE_PRESENT )
>          {
> -            /* We're replacing a non-SP page with a superpage.  Make sure to
> -             * handle freeing the table properly. */
> -            intermediate_entry = *p2m_entry;
> +            if ( flags & _PAGE_PSE )
> +            {
> +                iommu_old_flags =
> +                    p2m_get_iommu_flags(p2m_flags_to_type(flags));
> +                old_mfn = l1e_get_pfn(*p2m_entry);
> +            }
> +            else
> +            {
> +                iommu_old_flags = ~0;
> +                intermediate_entry = *p2m_entry;
> +            }
>          }
>  
>          ASSERT(!mfn_valid(mfn) || p2mt != p2m_mmio_direct);
> @@ -559,10 +578,7 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>          entry_content.l1 = l3e_content.l3;
>  
>          if ( entry_content.l1 != 0 )
> -        {
>              p2m_add_iommu_flags(&entry_content, 0, iommu_pte_flags);
> -            old_mfn = l1e_get_pfn(*p2m_entry);
> -        }
>  
>          p2m->write_p2m_entry(p2m, gfn, p2m_entry, entry_content, 3);
>          /* NB: paging_write_p2m_entry() handles tlb flushes properly */
> @@ -587,7 +603,10 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>          p2m_entry = p2m_find_entry(table, &gfn_remainder, gfn,
>                                     0, L1_PAGETABLE_ENTRIES);
>          ASSERT(p2m_entry);
> -        
> +        iommu_old_flags =
> +            
> p2m_get_iommu_flags(p2m_flags_to_type(l1e_get_flags(*p2m_entry)));
> +        old_mfn = l1e_get_pfn(*p2m_entry);
> +
>          if ( mfn_valid(mfn) || (p2mt == p2m_mmio_direct)
>                              || p2m_is_paging(p2mt) )
>              entry_content = p2m_l1e_from_pfn(mfn_x(mfn),
> @@ -596,10 +615,8 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>              entry_content = l1e_empty();
>  
>          if ( entry_content.l1 != 0 )
> -        {
>              p2m_add_iommu_flags(&entry_content, 0, iommu_pte_flags);
> -            old_mfn = l1e_get_pfn(*p2m_entry);
> -        }
> +
>          /* level 1 entry */
>          p2m->write_p2m_entry(p2m, gfn, p2m_entry, entry_content, 1);
>          /* NB: paging_write_p2m_entry() handles tlb flushes properly */
> @@ -610,14 +627,20 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>                                     L2_PAGETABLE_SHIFT - PAGE_SHIFT,
>                                     L2_PAGETABLE_ENTRIES);
>          ASSERT(p2m_entry);
> -        
> -        /* FIXME: Deal with 4k replaced by 2meg pages */
> -        if ( (l1e_get_flags(*p2m_entry) & _PAGE_PRESENT) &&
> -             !(l1e_get_flags(*p2m_entry) & _PAGE_PSE) )
> -        {
> -            /* We're replacing a non-SP page with a superpage.  Make sure to
> -             * handle freeing the table properly. */
> -            intermediate_entry = *p2m_entry;
> +        flags = l1e_get_flags(*p2m_entry);
> +        if ( flags & _PAGE_PRESENT )
> +        {
> +            if ( flags & _PAGE_PSE )
> +            {
> +                iommu_old_flags =
> +                    p2m_get_iommu_flags(p2m_flags_to_type(flags));
> +                old_mfn = l1e_get_pfn(*p2m_entry);
> +            }
> +            else
> +            {
> +                iommu_old_flags = ~0;
> +                intermediate_entry = *p2m_entry;
> +            }
>          }
>          
>          ASSERT(!mfn_valid(mfn) || p2mt != p2m_mmio_direct);
> @@ -631,10 +654,7 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>          entry_content.l1 = l2e_content.l2;
>  
>          if ( entry_content.l1 != 0 )
> -        {
>              p2m_add_iommu_flags(&entry_content, 0, iommu_pte_flags);
> -            old_mfn = l1e_get_pfn(*p2m_entry);
> -        }
>  
>          p2m->write_p2m_entry(p2m, gfn, p2m_entry, entry_content, 2);
>          /* NB: paging_write_p2m_entry() handles tlb flushes properly */
> @@ -645,11 +665,12 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
>           && (gfn + (1UL << page_order) - 1 > p2m->max_mapped_pfn) )
>          p2m->max_mapped_pfn = gfn + (1UL << page_order) - 1;
>  
> -    if ( iommu_enabled && need_iommu(p2m->domain) )
> +    if ( iommu_enabled && need_iommu(p2m->domain) &&
> +         (iommu_old_flags != iommu_pte_flags || old_mfn != mfn_x(mfn)) )
>      {
>          if ( iommu_use_hap_pt(p2m->domain) )
>          {
> -            if ( old_mfn && (old_mfn != mfn_x(mfn)) )
> +            if ( iommu_old_flags )
>                  amd_iommu_flush_pages(p2m->domain, gfn, page_order);

iommu_hap_pt_share is hardwired to 0 on AMD, making this if() clause
effectively dead.  Is this what you mean by your second TBD?  I would
suggest dropping it.

>          }
>          else
>
>

In this else clause there is a now-shadowed "flags" which might better
be renamed to iommu_flags to avoid confusion.

There is also an extra shadowed 'i' which could do with removing, as it
introduces a 64bit->32bit truncation (which is not currently a problem).

~Andrew

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