Re: [PATCH v1 02/10] powerpc/kernel/iommu: Align size for IOMMU_PAGE_SIZE on iommu_*_coherent()

Alexey Kardashevskiy
2020-08-31T00:47:38Z

On 29/08/2020 06:41, Leonardo Bras wrote:
> On Fri, 2020-08-28 at 11:40 +1000, Alexey Kardashevskiy wrote:
>>> I think it would be better to keep the code as much generic as possible
>>> regarding page sizes.
>>
>> Then you need to test it. Does 4K guest even boot (it should but I would
>> not bet much on it)?
>
> Maybe testing with host 64k pagesize and IOMMU 16MB pagesize in qemu
> should be enough, is there any chance to get indirect mapping in qemu
> like this? (DDW but with smaller DMA window available)

You will have to hack the guest kernel to always do indirect mapping or
hack QEMU's rtas_ibm_query_pe_dma_window() to return a small number of
available TCEs. But you will be testing QEMU/KVM which behave quite
differently to pHyp in this particular case.

>>>> Because if we want the former (==support), then we'll have to align the
>>>> size up to the bigger page size when allocating/zeroing system pages,
>>>> etc.
>>>
>>> This part I don't understand. Why do we need to align everything to the
>>> bigger pagesize?
>>>
>>> I mean, is not that enough that the range [ret, ret + size[ is both
>>> allocated by mm and mapped on a iommu range?
>>>
>>> Suppose a iommu_alloc_coherent() of 16kB on PAGESIZE = 4k and
>>> IOMMU_PAGE_SIZE() == 64k.
>>> Why 4 * cpu_pages mapped by a 64k IOMMU page is not enough?
>>> All the space the user asked for is allocated and mapped for DMA.
>>
>> The user asked to map 16K, the rest - 48K - is used for something else
>> (may be even mapped to another device) but you are making all 64K
>> accessible by the device which only should be able to access 16K.
>>
>> In practice, if this happens, H_PUT_TCE will simply fail.
>
> I have noticed mlx5 driver getting a few bytes in a buffer, and using
> iommu_map_page(). It does map a whole page for as few bytes as the user

Whole 4K system page or whole 64K iommu page?

> wants mapped, and the other bytes get used for something else, or just
> mapped on another DMA page.
> It seems to work fine.

With 4K system page and 64K IOMMU page? In practice it would take an
effort or/and bad luck to see it crashing. Thanks,

>
>>
>>
>>>> Bigger pages are not the case here as I understand it.
>>>
>>> I did not get this part, what do you mean?
>>
>> Possible IOMMU page sizes are 4K, 64K, 2M, 16M, 256M, 1GB, and the
>> supported set of sizes is different for P8/P9 and type of IO (PHB,
>> NVLink/CAPI).
>>
>>
>>>>> Update those functions to guarantee alignment with requested size
>>>>> using IOMMU_PAGE_ALIGN() before doing iommu_alloc() / iommu_free().
>>>>>
>>>>> Also, on iommu_range_alloc(), replace ALIGN(n, 1 << tbl->it_page_shift)
>>>>> with IOMMU_PAGE_ALIGN(n, tbl), which seems easier to read.
>>>>>
>>>>> Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
>>>>> ---
>>>>> arch/powerpc/kernel/iommu.c | 17 +++++++++--------
>>>>> 1 file changed, 9 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
>>>>> index 9704f3f76e63..d7086087830f 100644
>>>>> --- a/arch/powerpc/kernel/iommu.c
>>>>> +++ b/arch/powerpc/kernel/iommu.c
>>>>> @@ -237,10 +237,9 @@ static unsigned long iommu_range_alloc(struct device *dev,
>>>>> }
>>>>>
>>>>> if (dev)
>>>>> - boundary_size = ALIGN(dma_get_seg_boundary(dev) + 1,
>>>>> - 1 << tbl->it_page_shift);
>>>>> + boundary_size = IOMMU_PAGE_ALIGN(dma_get_seg_boundary(dev) + 1, tbl);
>>>>
>>>> Run checkpatch.pl, should complain about a long line.
>>>
>>> It's 86 columns long, which is less than the new limit of 100 columns
>>> Linus announced a few weeks ago. checkpatch.pl was updated too:
>>> https://www.phoronix.com/scan.php?page=news_item&px=Linux-Kernel-Deprecates-80-Col
>>
>> Yay finally :) Thanks,
>
> :)
>
>>
>>
>>>>
>>>>> else
>>>>> - boundary_size = ALIGN(1UL << 32, 1 << tbl->it_page_shift);
>>>>> + boundary_size = IOMMU_PAGE_ALIGN(1UL << 32, tbl);
>>>>> /* 4GB boundary for iseries_hv_alloc and iseries_hv_map */
>>>>>
>>>>> n = iommu_area_alloc(tbl->it_map, limit, start, npages, tbl->it_offset,
>>>>> @@ -858,6 +857,7 @@ void *iommu_alloc_coherent(struct device *dev, struct iommu_table *tbl,
>>>>> unsigned int order;
>>>>> unsigned int nio_pages, io_order;
>>>>> struct page *page;
>>>>> + size_t size_io = size;
>>>>>
>>>>> size = PAGE_ALIGN(size);
>>>>> order = get_order(size);
>>>>> @@ -884,8 +884,9 @@ void *iommu_alloc_coherent(struct device *dev, struct iommu_table *tbl,
>>>>> memset(ret, 0, size);
>>>>>
>>>>> /* Set up tces to cover the allocated range */
>>>>> - nio_pages = size >> tbl->it_page_shift;
>>>>> - io_order = get_iommu_order(size, tbl);
>>>>> + size_io = IOMMU_PAGE_ALIGN(size_io, tbl);
>>>>> + nio_pages = size_io >> tbl->it_page_shift;
>>>>> + io_order = get_iommu_order(size_io, tbl);
>>>>> mapping = iommu_alloc(dev, tbl, ret, nio_pages, DMA_BIDIRECTIONAL,
>>>>> mask >> tbl->it_page_shift, io_order, 0);
>>>>> if (mapping == DMA_MAPPING_ERROR) {
>>>>> @@ -900,11 +901,11 @@ void iommu_free_coherent(struct iommu_table *tbl, size_t size,
>>>>> void *vaddr, dma_addr_t dma_handle)
>>>>> {
>>>>> if (tbl) {
>>>>> - unsigned int nio_pages;
>>>>> + size_t size_io = IOMMU_PAGE_ALIGN(size, tbl);
>>>>> + unsigned int nio_pages = size_io >> tbl->it_page_shift;
>>>>>
>>>>> - size = PAGE_ALIGN(size);
>>>>> - nio_pages = size >> tbl->it_page_shift;
>>>>> iommu_free(tbl, dma_handle, nio_pages);
>>>>> +
>>>>
>>>> Unrelated new line.
>>>
>>> Will be removed. Thanks!
>>>
>>>>
>>>>> size = PAGE_ALIGN(size);
>>>>> free_pages((unsigned long)vaddr, get_order(size));
>>>>> }
>>>>>
>

--
Alexey