Subject: Re: [PATCH kernel v8 07/31] vfio: powerpc/spapr: Moving pinning/unpinning to helpers

From: Alexey Kardashevskiy <[email-masked]>
Date: Wed, 15 Apr 2015 22:09:46 +1000

On 04/15/2015 05:10 PM, David Gibson wrote:
> On Fri, Apr 10, 2015 at 04:30:49PM +1000, Alexey Kardashevskiy wrote:
>> This is a pretty mechanical patch to make next patches simpler.
>>
>> New tce_iommu_unuse_page() helper does put_page() now but it might skip
>> that after the memory registering patch applied.
>>
>> As we are here, this removes unnecessary checks for a value returned
>> by pfn_to_page() as it cannot possibly return NULL.
>>
>> This moves tce_iommu_disable() later to let tce_iommu_clear() know if
>> the container has been enabled because if it has not been, then
>> put_page() must not be called on TCEs from the TCE table. This situation
>> is not yet possible but it will after KVM acceleration patchset is
>> applied.
>>
>> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
>> ---
>> Changes:
>> v6:
>> * tce_get_hva() returns hva via a pointer
>> ---
>>   drivers/vfio/vfio_iommu_spapr_tce.c | 68 +++++++++++++++++++++++++++----------
>>   1 file changed, 50 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
>> index c137bb3..ec5ee83 100644
>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
>> @@ -196,7 +196,6 @@ static void tce_iommu_release(void *iommu_data)
>>   	struct iommu_table *tbl = container->tbl;
>>
>>   	WARN_ON(tbl && !tbl->it_group);
>> -	tce_iommu_disable(container);
>>
>>   	if (tbl) {
>>   		tce_iommu_clear(container, tbl,	tbl->it_offset, tbl->it_size);
>> @@ -204,63 +203,96 @@ static void tce_iommu_release(void *iommu_data)
>>   		if (tbl->it_group)
>>   			tce_iommu_detach_group(iommu_data, tbl->it_group);
>>   	}
>> +
>> +	tce_iommu_disable(container);
>> +
>>   	mutex_destroy(&container->lock);
>>
>>   	kfree(container);
>>   }
>>
>> +static void tce_iommu_unuse_page(struct tce_container *container,
>> +		unsigned long oldtce)
>> +{
>> +	struct page *page;
>> +
>> +	if (!(oldtce & (TCE_PCI_READ | TCE_PCI_WRITE)))
>> +		return;
>> +
>> +	/*
>> +	 * VFIO cannot map/unmap when a container is not enabled so
>> +	 * we would not need this check but KVM could map/unmap and if
>> +	 * this happened, we must not put pages as KVM does not get them as
>> +	 * it expects memory pre-registation to do this part.
>> +	 */
>> +	if (!container->enabled)
>> +		return;
>
> This worries me a bit.  How can whether the contained is enabled now
> safely tell you whether get_page() at some earlier point in time?

This is a leftover, I'll remove it as after the "iommu v2" patch there will 
be tce_iommu_unuse_page_v2().

>> +
>> +	page = pfn_to_page(__pa(oldtce) >> PAGE_SHIFT);
>> +
>> +	if (oldtce & TCE_PCI_WRITE)
>> +		SetPageDirty(page);
>> +
>> +	put_page(page);
>> +}
>> +
>>   static int tce_iommu_clear(struct tce_container *container,
>>   		struct iommu_table *tbl,
>>   		unsigned long entry, unsigned long pages)
>>   {
>>   	unsigned long oldtce;
>> -	struct page *page;
>>
>>   	for ( ; pages; --pages, ++entry) {
>>   		oldtce = iommu_clear_tce(tbl, entry);
>>   		if (!oldtce)
>>   			continue;
>>
>> -		page = pfn_to_page(oldtce >> PAGE_SHIFT);
>> -		WARN_ON(!page);
>> -		if (page) {
>> -			if (oldtce & TCE_PCI_WRITE)
>> -				SetPageDirty(page);
>> -			put_page(page);
>> -		}
>> +		tce_iommu_unuse_page(container, (unsigned long) __va(oldtce));
>>   	}
>>
>>   	return 0;
>>   }
>>
>> +static int tce_get_hva(unsigned long tce, unsigned long *hva)
>> +{
>> +	struct page *page = NULL;
>> +	enum dma_data_direction direction = iommu_tce_direction(tce);
>> +
>> +	if (get_user_pages_fast(tce & PAGE_MASK, 1,
>> +			direction != DMA_TO_DEVICE, &page) != 1)
>> +		return -EFAULT;
>> +
>> +	*hva = (unsigned long) page_address(page);
>> +
>> +	return 0;
>> +}
>
> I'd prefer to see this called tce_iommu_use_page() for symmetry.

If I rename this one, then what would I call tce_get_hva_cached() from 
"fio: powerpc/spapr: Register memory and define IOMMU v2"?

>
>> +
>>   static long tce_iommu_build(struct tce_container *container,
>>   		struct iommu_table *tbl,
>>   		unsigned long entry, unsigned long tce, unsigned long pages)
>>   {
>>   	long i, ret = 0;
>> -	struct page *page = NULL;
>> +	struct page *page;
>>   	unsigned long hva;
>>   	enum dma_data_direction direction = iommu_tce_direction(tce);
>>
>>   	for (i = 0; i < pages; ++i) {
>> -		ret = get_user_pages_fast(tce & PAGE_MASK, 1,
>> -				direction != DMA_TO_DEVICE, &page);
>> -		if (unlikely(ret != 1)) {
>> -			ret = -EFAULT;
>> +		ret = tce_get_hva(tce, &hva);
>> +		if (ret)
>>   			break;
>> -		}
>>
>> +		page = pfn_to_page(__pa(hva) >> PAGE_SHIFT);
>>   		if (!tce_page_is_contained(page, tbl->it_page_shift)) {
>>   			ret = -EPERM;
>>   			break;
>>   		}
>>
>> -		hva = (unsigned long) page_address(page) +
>> -			(tce & IOMMU_PAGE_MASK(tbl) & ~PAGE_MASK);
>> +		/* Preserve offset within IOMMU page */
>> +		hva |= tce & IOMMU_PAGE_MASK(tbl) & ~PAGE_MASK;
>>
>>   		ret = iommu_tce_build(tbl, entry + i, hva, direction);
>>   		if (ret) {
>> -			put_page(page);
>> +			tce_iommu_unuse_page(container, hva);
>>   			pr_err("iommu_tce: %s failed ioba=%lx, tce=%lx, ret=%ld\n",
>>   					__func__, entry << tbl->it_page_shift,
>>   					tce, ret);
>

-- 
Alexey