Subject: Re: [PATCH kernel v7 04/31] vfio: powerpc/spapr: Use it_page_size

From: Alex Williamson <[email-masked]>
Date: Wed, 01 Apr 2015 20:50:33 -0600

On Thu, 2015-04-02 at 13:30 +1100, Alexey Kardashevskiy wrote:
> On 04/02/2015 08:48 AM, Alex Williamson wrote:
> > On Sat, 2015-03-28 at 01:54 +1100, Alexey Kardashevskiy wrote:
> >> This makes use of the it_page_size from the iommu_table struct
> >> as page size can differ.
> >>
> >> This replaces missing IOMMU_PAGE_SHIFT macro in commented debug code
> >> as recently introduced IOMMU_PAGE_XXX macros do not include
> >> IOMMU_PAGE_SHIFT.
> >>
> >> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> >> Reviewed-by: David Gibson <[email-masked]>
> >> ---
> >>   drivers/vfio/vfio_iommu_spapr_tce.c | 26 +++++++++++++-------------
> >>   1 file changed, 13 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> >> index f835e63..8bbee22 100644
> >> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> >> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> >> @@ -91,7 +91,7 @@ static int tce_iommu_enable(struct tce_container *container)
> >>   	 * enforcing the limit based on the max that the guest can map.
> >>   	 */
> >>   	down_write(&current->mm->mmap_sem);
> >> -	npages = (tbl->it_size << IOMMU_PAGE_SHIFT_4K) >> PAGE_SHIFT;
> >> +	npages = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
> >>   	locked = current->mm->locked_vm + npages;
> >>   	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> >>   	if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
> >> @@ -120,7 +120,7 @@ static void tce_iommu_disable(struct tce_container *container)
> >>
> >>   	down_write(&current->mm->mmap_sem);
> >>   	current->mm->locked_vm -= (container->tbl->it_size <<
> >> -			IOMMU_PAGE_SHIFT_4K) >> PAGE_SHIFT;
> >> +			container->tbl->it_page_shift) >> PAGE_SHIFT;
> >>   	up_write(&current->mm->mmap_sem);
> >>   }
> >>
> >> @@ -222,7 +222,7 @@ static long tce_iommu_build(struct tce_container *container,
> >>   					tce, ret);
> >>   			break;
> >>   		}
> >> -		tce += IOMMU_PAGE_SIZE_4K;
> >> +		tce += IOMMU_PAGE_SIZE(tbl);
> >
> >
> > Is PAGE_SIZE ever smaller than IOMMU_PAGE_SIZE(tbl)?  IOW, can the page
> > we got from get_user_pages_fast() ever not completely fill the tce
> > entry?
> 
> 
> Yes. IOMMU_PAGE_SIZE is 4K/64K/16M (16M is with huge pages enabled in QEMU 
> with -mempath), PAGE_SIZE is 4K/64K (normally 64K).

Isn't that a problem then that you're filling the tce with processor
page sizes via get_user_pages_fast(), but incrementing the tce by by
IOMMU page size?  For example, if PAGE_SIZE = 4K and IOMMU_PAGE_SIZE !=
4K have we really pinned all of the memory backed by the tce?  Where do
you make sure the 4K page is really contiguous for the IOMMU page?