RE: [EXTERNAL] [PATCH v4 08/14] remove remaining uses of iommu_legacy_map/unmap

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: 06 August 2020 11:29
> To: Paul Durrant <paul@xen.org>
> Cc: xen-devel@lists.xenproject.org; Durrant, Paul <pdurrant@amazon.co.uk>; Andrew Cooper
> <andrew.cooper3@citrix.com>; Wei Liu <wl@xen.org>; Roger Pau Monné <roger.pau@citrix.com>; George
> Dunlap <george.dunlap@citrix.com>; Ian Jackson <ian.jackson@eu.citrix.com>; Julien Grall
> <julien@xen.org>; Stefano Stabellini <sstabellini@kernel.org>; Jun Nakajima <jun.nakajima@intel.com>;
> Kevin Tian <kevin.tian@intel.com>
> Subject: RE: [EXTERNAL] [PATCH v4 08/14] remove remaining uses of iommu_legacy_map/unmap
> 
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> 
> 
> On 04.08.2020 15:42, Paul Durrant wrote:
> > The 'legacy' functions do implicit flushing so amend the callers to do the
> > appropriate flushing.
> >
> > Unfortunately, because of the structure of the P2M code, we cannot remove
> > the per-CPU 'iommu_dont_flush_iotlb' global and the optimization it
> > facilitates. It is now checked directly iommu_iotlb_flush(). Also, it is
> > now declared as bool (rather than bool_t) and setting/clearing it are no
> > longer pointlessly gated on is_iommu_enabled() returning true. (Arguably
> > it is also pointless to gate the call to iommu_iotlb_flush() on that
> > condition - since it is a no-op in that case - but the if clause allows
> > the scope of a stack variable to be restricted).
> >
> > NOTE: The code in memory_add() now fails if the number of pages passed to
> >       a single call overflows an unsigned int. I don't believe this will
> >       ever happen in practice.
> 
> I.e. you don't think adding 16Tb of memory in one go is possible?
> I wouldn't bet on that ...
> 

I've re-worked previous patches to use unsigned long so I don't need this restriction any more.

> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -2446,10 +2446,16 @@ static int cleanup_page_mappings(struct page_info *page)
> >
> >          if ( d && unlikely(need_iommu_pt_sync(d)) && is_pv_domain(d) )
> >          {
> > -            int rc2 = iommu_legacy_unmap(d, _dfn(mfn), PAGE_ORDER_4K);
> > +            unsigned int flush_flags = 0;
> > +            int err;
> >
> > +            err = iommu_unmap(d, _dfn(mfn), PAGE_ORDER_4K, 1, &flush_flags);
> >              if ( !rc )
> > -                rc = rc2;
> > +                rc = err;
> > +
> > +            err = iommu_iotlb_flush(d, _dfn(mfn), PAGE_ORDER_4K, 1, flush_flags);
> > +            if ( !rc )
> > +                rc = err;
> >          }
> 
> Wasn't the earlier change to add flushing in the error case to
> allow to simplify code like this to
> 
>         if ( d && unlikely(need_iommu_pt_sync(d)) && is_pv_domain(d) )
>         {
>             unsigned int flush_flags = 0;
>             int err;
> 
>             err = iommu_unmap(d, _dfn(mfn), PAGE_ORDER_4K, 1, &flush_flags);
>             if ( !err )
>                 err = iommu_iotlb_flush(d, _dfn(mfn), PAGE_ORDER_4K, 1, flush_flags);
>             if ( !rc )
>                 rc = err;
>         }
> 
> ?

Yes.

> 
> > @@ -1441,9 +1446,16 @@ int clear_identity_p2m_entry(struct domain *d, unsigned long gfn_l)
> >
> >      if ( !paging_mode_translate(d) )
> >      {
> > -        if ( !is_iommu_enabled(d) )
> > -            return 0;
> > -        return iommu_legacy_unmap(d, _dfn(gfn_l), PAGE_ORDER_4K);
> > +        unsigned int flush_flags = 0;
> > +        int err;
> > +
> > +        ret = iommu_unmap(d, _dfn(gfn_l), PAGE_ORDER_4K, 1, &flush_flags);
> > +
> > +        err = iommu_iotlb_flush(d, _dfn(gfn_l), PAGE_ORDER_4K, 1, flush_flags);
> > +        if ( !ret )
> > +            ret = err;
> > +
> > +        return ret;
> >      }
> 
> Similarly here then.
> 

Yes.

> > --- a/xen/arch/x86/x86_64/mm.c
> > +++ b/xen/arch/x86/x86_64/mm.c
> > @@ -1413,21 +1413,22 @@ int memory_add(unsigned long spfn, unsigned long epfn, unsigned int pxm)
> >           !iommu_use_hap_pt(hardware_domain) &&
> >           !need_iommu_pt_sync(hardware_domain) )
> >      {
> > -        for ( i = spfn; i < epfn; i++ )
> > -            if ( iommu_legacy_map(hardware_domain, _dfn(i), _mfn(i),
> > -                                  PAGE_ORDER_4K,
> > -                                  IOMMUF_readable | IOMMUF_writable) )
> > -                break;
> > -        if ( i != epfn )
> > -        {
> > -            while (i-- > old_max)
> > -                /* If statement to satisfy __must_check. */
> > -                if ( iommu_legacy_unmap(hardware_domain, _dfn(i),
> > -                                        PAGE_ORDER_4K) )
> > -                    continue;
> > +        unsigned int flush_flags = 0;
> > +        unsigned int n = epfn - spfn;
> > +        int rc;
> >
> > +        ret = -EOVERFLOW;
> > +        if ( spfn + n != epfn )
> > +            goto destroy_m2p;
> > +
> > +        rc = iommu_map(hardware_domain, _dfn(i), _mfn(i),
> > +                       PAGE_ORDER_4K, n, IOMMUF_readable | IOMMUF_writable,
> > +                       &flush_flags);
> > +        if ( !rc )
> > +            rc = iommu_iotlb_flush(hardware_domain, _dfn(i), PAGE_ORDER_4K, n,
> > +                                       flush_flags);
> > +        if ( rc )
> >              goto destroy_m2p;
> > -        }
> >      }
> 
> Did you mean to use "ret" here instead of introducing "rc"?
> 

The previous code did not set ret in the case of an iommu op failure but that does appear to be a mistake. I will use ret, as you suggest, but I will call it out in the commit description too.

> > --- a/xen/common/grant_table.c
> > +++ b/xen/common/grant_table.c
> > @@ -1225,11 +1225,23 @@ map_grant_ref(
> >              kind = IOMMUF_readable;
> >          else
> >              kind = 0;
> > -        if ( kind && iommu_legacy_map(ld, _dfn(mfn_x(mfn)), mfn, 0, kind) )
> > +        if ( kind )
> >          {
> > -            double_gt_unlock(lgt, rgt);
> > -            rc = GNTST_general_error;
> > -            goto undo_out;
> > +            dfn_t dfn = _dfn(mfn_x(mfn));
> > +            unsigned int flush_flags = 0;
> > +            int err;
> > +
> > +            err = iommu_map(ld, dfn, mfn, 0, 1, kind, &flush_flags);
> > +            if ( !err )
> > +                err = iommu_iotlb_flush(ld, dfn, 0, 1, flush_flags);
> 
> Question of 0 vs PAGE_ORDER_4K again.
> 
> > @@ -1473,21 +1485,25 @@ unmap_common(
> >      if ( rc == GNTST_okay && gnttab_need_iommu_mapping(ld) )
> >      {
> >          unsigned int kind;
> > +        dfn_t dfn = _dfn(mfn_x(op->mfn));
> > +        unsigned int flush_flags = 0;
> >          int err = 0;
> >
> >          double_gt_lock(lgt, rgt);
> >
> >          kind = mapkind(lgt, rd, op->mfn);
> >          if ( !kind )
> > -            err = iommu_legacy_unmap(ld, _dfn(mfn_x(op->mfn)), 0);
> > +            err = iommu_unmap(ld, dfn, 0, 1, &flush_flags);
> >          else if ( !(kind & MAPKIND_WRITE) )
> > -            err = iommu_legacy_map(ld, _dfn(mfn_x(op->mfn)), op->mfn, 0,
> > -                                   IOMMUF_readable);
> > -
> > -        double_gt_unlock(lgt, rgt);
> > +            err = iommu_map(ld, dfn, op->mfn, 0, 1, IOMMUF_readable,
> > +                            &flush_flags);
> >
> > +        if ( !err )
> > +            err = iommu_iotlb_flush(ld, dfn, 0, 1, flush_flags);
> >          if ( err )
> >              rc = GNTST_general_error;
> > +
> > +        double_gt_unlock(lgt, rgt);
> >      }
> 
> While moving the unlock ahead of the flush would be somewhat troublesome
> in the map case, it seems straightforward here. Even if this gets further
> adjusted by a later patch, it should imo be done here - the later patch
> may also go in much later.
> 

Ok.

> > --- a/xen/common/memory.c
> > +++ b/xen/common/memory.c
> > @@ -824,8 +824,7 @@ int xenmem_add_to_physmap(struct domain *d, struct xen_add_to_physmap *xatp,
> >      xatp->gpfn += start;
> >      xatp->size -= start;
> >
> > -    if ( is_iommu_enabled(d) )
> > -       this_cpu(iommu_dont_flush_iotlb) = 1;
> > +    this_cpu(iommu_dont_flush_iotlb) = true;
> 
> Just like you replace the original instance here, ...
> 
> > @@ -845,6 +844,8 @@ int xenmem_add_to_physmap(struct domain *d, struct xen_add_to_physmap *xatp,
> >          }
> >      }
> >
> > +    this_cpu(iommu_dont_flush_iotlb) = false;
> > +
> >      if ( is_iommu_enabled(d) )
> >      {
> >          int ret;
> 
> ... I'm sure you meant to also remove the original instance from
> down below here.

I did indeed. Thanks for spotting.

> 
> > @@ -364,7 +341,7 @@ int iommu_iotlb_flush(struct domain *d, dfn_t dfn, unsigned int page_order,
> >      int rc;
> >
> >      if ( !is_iommu_enabled(d) || !hd->platform_ops->iotlb_flush ||
> > -         !page_count || !flush_flags )
> > +         !page_count || !flush_flags || this_cpu(iommu_dont_flush_iotlb) )
> >          return 0;
> 
> The patch description ought to assure the safety of this change: So
> far, despite the flag set callers of iommu_iotlb_flush() (which
> may be unaware of the flag's state) did get what they did ask for.
> The change relies on there not being any such uses.
> 

Ok, I'll call it out.

> > @@ -370,15 +362,12 @@ void iommu_dev_iotlb_flush_timeout(struct domain *d, struct pci_dev *pdev);
> >
> >  /*
> >   * The purpose of the iommu_dont_flush_iotlb optional cpu flag is to
> > - * avoid unecessary iotlb_flush in the low level IOMMU code.
> > - *
> > - * iommu_map_page/iommu_unmap_page must flush the iotlb but somethimes
> > - * this operation can be really expensive. This flag will be set by the
> > - * caller to notify the low level IOMMU code to avoid the iotlb flushes.
> > - * iommu_iotlb_flush/iommu_iotlb_flush_all will be explicitly called by
> > - * the caller.
> > + * avoid unecessary IOMMU flushing while updating the P2M.
> 
> Correct the spelling of "unnecessary" at the same time?
> 

Oh yes. Will do.

  Paul

> Jan