RE: [PATCH] IOMMU: avoid double flushing in shared page table case

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Tuesday, October 20, 2020 9:53 PM
> 
> While the flush coalescing optimization has been helping the non-shared
> case, it has actually lead to double flushes in the shared case (which
> ought to be the more common one nowadays at least): Once from
> *_set_entry() and a second time up the call tree from wherever the
> overriding flag gets played with. In alignment with XSA-346 suppress
> flushing in this case.
> 
> Similarly avoid excessive setting of IOMMU_FLUSHF_added on the batched
> flushes: "idx" hasn't been added a new mapping for.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> ---
> TBD: The Arm part really is just for completeness (and hence could also
>      be dropped) - the affected mapping spaces aren't currently
>      supported there.
> 
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1045,7 +1045,7 @@ static int __p2m_set_entry(struct p2m_do
>          p2m->lowest_mapped_gfn = gfn_min(p2m->lowest_mapped_gfn, sgfn);
>      }
> 
> -    if ( is_iommu_enabled(p2m->domain) &&
> +    if ( is_iommu_enabled(p2m->domain)
> && !this_cpu(iommu_dont_flush_iotlb) &&
>           (lpae_is_valid(orig_pte) || lpae_is_valid(*entry)) )
>      {
>          unsigned int flush_flags = 0;
> --- a/xen/arch/x86/mm/p2m-ept.c
> +++ b/xen/arch/x86/mm/p2m-ept.c
> @@ -842,7 +842,7 @@ out:
>      if ( rc == 0 && p2m_is_hostp2m(p2m) &&
>           need_modify_vtd_table )
>      {
> -        if ( iommu_use_hap_pt(d) )
> +        if ( iommu_use_hap_pt(d) && !this_cpu(iommu_dont_flush_iotlb) )
>              rc = iommu_iotlb_flush(d, _dfn(gfn), 1ul << order,
>                                     (iommu_flags ? IOMMU_FLUSHF_added : 0) |
>                                     (vtd_pte_present ? IOMMU_FLUSHF_modified
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -870,7 +870,7 @@ int xenmem_add_to_physmap(struct domain
>          this_cpu(iommu_dont_flush_iotlb) = 0;
> 
>          ret = iommu_iotlb_flush(d, _dfn(xatp->idx - done), done,
> -                                IOMMU_FLUSHF_added | IOMMU_FLUSHF_modified);
> +                                IOMMU_FLUSHF_modified);
>          if ( unlikely(ret) && rc >= 0 )
>              rc = ret;
>