linux-kernel - Re: [PATCH kernel v7 04/31] vfio: powerpc/spapr: Use it_page

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Message-ID: <1427946378.5567.340.camel@redhat.com>
Date:	Wed, 01 Apr 2015 21:46:18 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
Cc:	linuxppc-dev@...ts.ozlabs.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel v7 04/31] vfio: powerpc/spapr: Use it_page_size

On Thu, 2015-04-02 at 14:40 +1100, Alexey Kardashevskiy wrote:
> On 04/02/2015 01:50 PM, Alex Williamson wrote:
> > 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 <aik@...abs.ru>
> >>>> Reviewed-by: David Gibson <david@...son.dropbear.id.au>
> >>>> ---
> >>>>    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?
> 
> 
> Aaaah. This is just not supported. Instead, after the previous patch 
> ("vfio: powerpc/spapr: Check that TCE page size is equal to it_page_size", 
> which need fixed subject), tce_page_is_contained(page4K, 64K) will return 
> false and the caller - tce_iommu_build() - will return -EPERM.

Ok, makes sense.  Thanks

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