Re: [PATCH v2 6/6] vfio/type1: Use mapping page mask for pfnmaps

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 18 Feb 2025 20:36:22 -0600
Mitchell Augustin <mitchell.augustin@canonical.com> wrote:

> /s/follow_pfnmap_args.pgmask/follow_pfnmap_args.addr_mask/ in v2
> commit log.

Thanks for spotting that, if there's no other cause for a re-spin I'll
fix that on commit.  Thanks for the review and testing!

Alex

> Aside from that, it works as expected.
> 
> Reviewed-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>
> Tested-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>
> 
> 
> On Tue, Feb 18, 2025 at 5:14 PM Alex Williamson
> <alex.williamson@redhat.com> wrote:
> >
> > On Tue, 18 Feb 2025 17:47:46 -0500
> > Peter Xu <peterx@redhat.com> wrote:
> >  
> > > On Tue, Feb 18, 2025 at 03:22:06PM -0700, Alex Williamson wrote:  
> > > > vfio-pci supports huge_fault for PCI MMIO BARs and will insert pud and
> > > > pmd mappings for well aligned mappings.  follow_pfnmap_start() walks the
> > > > page table and therefore knows the page mask of the level where the
> > > > address is found and returns this through follow_pfnmap_args.pgmask.
> > > > Subsequent pfns from this address until the end of the mapping page are
> > > > necessarily consecutive.  Use this information to retrieve a range of
> > > > pfnmap pfns in a single pass.
> > > >
> > > > With optimal mappings and alignment on systems with 1GB pud and 4KB
> > > > page size, this reduces iterations for DMA mapping PCI BARs by a
> > > > factor of 256K.  In real world testing, the overhead of iterating
> > > > pfns for a VM DMA mapping a 32GB PCI BAR is reduced from ~1s to
> > > > sub-millisecond overhead.
> > > >
> > > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > > ---
> > > >  drivers/vfio/vfio_iommu_type1.c | 23 ++++++++++++++++-------
> > > >  1 file changed, 16 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > > index ce661f03f139..0ac56072af9f 100644
> > > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > > @@ -520,7 +520,7 @@ static void vfio_batch_fini(struct vfio_batch *batch)
> > > >
> > > >  static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> > > >                         unsigned long vaddr, unsigned long *pfn,
> > > > -                       bool write_fault)
> > > > +                       unsigned long *addr_mask, bool write_fault)
> > > >  {
> > > >     struct follow_pfnmap_args args = { .vma = vma, .address = vaddr };
> > > >     int ret;
> > > > @@ -544,10 +544,12 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> > > >                     return ret;
> > > >     }
> > > >
> > > > -   if (write_fault && !args.writable)
> > > > +   if (write_fault && !args.writable) {
> > > >             ret = -EFAULT;
> > > > -   else
> > > > +   } else {
> > > >             *pfn = args.pfn;
> > > > +           *addr_mask = args.addr_mask;
> > > > +   }
> > > >
> > > >     follow_pfnmap_end(&args);
> > > >     return ret;
> > > > @@ -590,15 +592,22 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
> > > >     vma = vma_lookup(mm, vaddr);
> > > >
> > > >     if (vma && vma->vm_flags & VM_PFNMAP) {
> > > > -           ret = follow_fault_pfn(vma, mm, vaddr, pfn, prot & IOMMU_WRITE);
> > > > +           unsigned long addr_mask;
> > > > +
> > > > +           ret = follow_fault_pfn(vma, mm, vaddr, pfn, &addr_mask,
> > > > +                                  prot & IOMMU_WRITE);
> > > >             if (ret == -EAGAIN)
> > > >                     goto retry;
> > > >
> > > >             if (!ret) {
> > > > -                   if (is_invalid_reserved_pfn(*pfn))
> > > > -                           ret = 1;
> > > > -                   else
> > > > +                   if (is_invalid_reserved_pfn(*pfn)) {
> > > > +                           unsigned long epfn;
> > > > +
> > > > +                           epfn = (*pfn | (~addr_mask >> PAGE_SHIFT)) + 1;
> > > > +                           ret = min_t(long, npages, epfn - *pfn);  
> > >
> > > s/long/unsigned long/?  
> >
> > ret is signed long since it's the function return and needs to be able
> > to return -errno, so long was the intention here.  Thanks,
> >
> > Alex
> >  
> > > Reviewed-by: Peter Xu <peterx@redhat.com>
> > >  
> > > > +                   } else {
> > > >                             ret = -EFAULT;
> > > > +                   }
> > > >             }
> > > >     }
> > > >  done:
> > > > --
> > > > 2.48.1
> > > >  
> > >  
> >  
> 
> 
> --
> Mitchell Augustin
> Software Engineer - Ubuntu Partner Engineering
>