RE: [PATCH v4 06/14] iommu: flush I/O TLB if iommu_map() or iommu_unmap() fail

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Tian, Kevin <kevin.tian@xxxxxxxxx>
> Sent: 14 August 2020 07:53
> To: Paul Durrant <paul@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Durrant, Paul <pdurrant@xxxxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>
> Subject: RE: [EXTERNAL] [PATCH v4 06/14] iommu: flush I/O TLB if iommu_map() 
> or iommu_unmap() fail
> 
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> 
> > From: Paul Durrant
> > Sent: Tuesday, August 4, 2020 9:42 PM
> >
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >
> > This patch adds a full I/O TLB flush to the error paths of iommu_map() and
> > iommu_unmap().
> >
> > Without this change callers need constructs such as:
> >
> > rc = iommu_map/unmap(...)
> > err = iommu_flush(...)
> > if ( !rc )
> >   rc = err;
> >
> > With this change, it can be simplified to:
> >
> > rc = iommu_map/unmap(...)
> > if ( !rc )
> >   rc = iommu_flush(...)
> >
> > because, if the map or unmap fails the flush will be unnecessary. This saves
> 
> this statement is different from change in iommu_map...
> 
> > a stack variable and generally makes the call sites tidier.
> >
> > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > ---
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> >
> > v2:
> >  - New in v2
> > ---
> >  xen/drivers/passthrough/iommu.c | 28 ++++++++++++----------------
> >  1 file changed, 12 insertions(+), 16 deletions(-)
> >
> > diff --git a/xen/drivers/passthrough/iommu.c
> > b/xen/drivers/passthrough/iommu.c
> > index 660dc5deb2..e2c0193a09 100644
> > --- a/xen/drivers/passthrough/iommu.c
> > +++ b/xen/drivers/passthrough/iommu.c
> > @@ -274,6 +274,10 @@ int iommu_map(struct domain *d, dfn_t dfn, mfn_t
> > mfn,
> >          break;
> >      }
> >
> > +    /* Something went wrong so flush everything and clear flush flags */
> > +    if ( unlikely(rc) && iommu_iotlb_flush_all(d, *flush_flags) )
> > +        flush_flags = 0;
> > +
> 
> ... earlier you said flush is unnecessary if map fails. But here actually you
> still need to flush everything so it's just sort of moving error-path flush
> within the map function?

Yes, that's actually what's happening. The language in the comment is ambiguous 
I guess. I'll modify it to say

"because, if the map or unmap fails an explicit flush will be unnecessary."

Hopefully that is clearer.

  Paul

> 
> Thanks
> Kevin
> 
> >      return rc;
> >  }
> >
> > @@ -283,14 +287,8 @@ int iommu_legacy_map(struct domain *d, dfn_t dfn,
> > mfn_t mfn,
> >      unsigned int flush_flags = 0;
> >      int rc = iommu_map(d, dfn, mfn, page_order, flags, &flush_flags);
> >
> > -    if ( !this_cpu(iommu_dont_flush_iotlb) )
> > -    {
> > -        int err = iommu_iotlb_flush(d, dfn, (1u << page_order),
> > -                                    flush_flags);
> > -
> > -        if ( !rc )
> > -            rc = err;
> > -    }
> > +    if ( !this_cpu(iommu_dont_flush_iotlb) && !rc )
> > +        rc = iommu_iotlb_flush(d, dfn, (1u << page_order), flush_flags);
> >
> >      return rc;
> >  }
> > @@ -330,6 +328,10 @@ int iommu_unmap(struct domain *d, dfn_t dfn,
> > unsigned int page_order,
> >          }
> >      }
> >
> > +    /* Something went wrong so flush everything and clear flush flags */
> > +    if ( unlikely(rc) && iommu_iotlb_flush_all(d, *flush_flags) )
> > +        flush_flags = 0;
> > +
> >      return rc;
> >  }
> >
> > @@ -338,14 +340,8 @@ int iommu_legacy_unmap(struct domain *d, dfn_t
> > dfn, unsigned int page_order)
> >      unsigned int flush_flags = 0;
> >      int rc = iommu_unmap(d, dfn, page_order, &flush_flags);
> >
> > -    if ( !this_cpu(iommu_dont_flush_iotlb) )
> > -    {
> > -        int err = iommu_iotlb_flush(d, dfn, (1u << page_order),
> > -                                    flush_flags);
> > -
> > -        if ( !rc )
> > -            rc = err;
> > -    }
> > +    if ( !this_cpu(iommu_dont_flush_iotlb) && ! rc )
> > +        rc = iommu_iotlb_flush(d, dfn, (1u << page_order), flush_flags);
> >
> >      return rc;
> >  }
> > --
> > 2.20.1
> >