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

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Message-ID: <20250218161407.6ae2b082.alex.williamson@redhat.com>
Date: Tue, 18 Feb 2025 16:14:07 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 mitchell.augustin@...onical.com, clg@...hat.com, jgg@...dia.com,
 willy@...radead.org
Subject: Re: [PATCH v2 6/6] vfio/type1: Use mapping page mask for pfnmaps

On Tue, 18 Feb 2025 17:47:46 -0500
Peter Xu <peterx@...hat.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@...hat.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@...hat.com>
> 
> > +			} else {
> >  				ret = -EFAULT;
> > +			}
> >  		}
> >  	}
> >  done:
> > -- 
> > 2.48.1
> >   
>