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

From: Alexey Kardashevskiy <[email-masked]>
Date: Thu, 02 Apr 2015 14:40:04 +1100

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 <[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?

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.

-- 
Alexey