RE: [PATCH v2 0.9/2] VT-d: don't needlessly look up DID

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

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Thursday, April 7, 2022 5:28 PM
> 
> If get_iommu_domid() in domain_context_unmap_one() fails, we better
> wouldn't clear the context entry in the first place, as we're then unable
> to issue the corresponding flush. However, we have no need to look up the
> DID in the first place: What needs flushing is very specifically the DID
> that was in the context entry before our clearing of it.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

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

> ---
> This (an intended follow-up to XSA-399) is actually a prereq to what was
> called patch 1 so far in this series.
> 
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -1830,18 +1830,12 @@ int domain_context_unmap_one(
>          return 0;
>      }
> 
> +    iommu_domid = context_domain_id(*context);
> +
>      context_clear_present(*context);
>      context_clear_entry(*context);
>      iommu_sync_cache(context, sizeof(struct context_entry));
> 
> -    iommu_domid = get_iommu_did(domid, iommu, !domain->is_dying);
> -    if ( iommu_domid == -1 )
> -    {
> -        spin_unlock(&iommu->lock);
> -        unmap_vtd_domain_page(context_entries);
> -        return -EINVAL;
> -    }
> -
>      rc = iommu_flush_context_device(iommu, iommu_domid,
>                                      PCI_BDF2(bus, devfn),
>                                      DMA_CCMD_MASK_NOBIT, 0);