ACK: [PATCH] UBUNTU: SAUCE: swiotlb: Max mapping size takes min align mask into account

[Tim Gardner]

On 5/12/22 07:07, Thadeu Lima de Souza Cascardo wrote:
> On Thu, May 12, 2022 at 06:07:06AM -0600, Tim Gardner wrote:
>> From: Tianyu Lan <Tianyu.Lan at microsoft.com>
>>
>> BugLink: https://bugs.launchpad.net/bugs/1973169
>>
>> swiotlb_find_slots() skips slots according to io tlb aligned mask
>> calculated from min aligned mask and original physical address
>> offset. This affects max mapping size. The mapping size can't
>> achieve the IO_TLB_SEGSIZE * IO_TLB_SIZE when original offset is
>> non-zero. This will cause system boot up failure in Hyper-V
>> Isolation VM where swiotlb force is enabled. Scsi layer use return
>> value of dma_max_mapping_size() to set max segment size and it
>> finally calls swiotlb_max_mapping_size(). Hyper-V storage driver
>> sets min align mask to 4k - 1. Scsi layer may pass 256k length of
>> request buffer with 0~4k offset and Hyper-V storage driver can't
>> get swiotlb bounce buffer via DMA API. Swiotlb_find_slots() can't
>> find 256k length bounce buffer with offset. Make swiotlb_max_mapping
>> _size() take min align mask into account.
>>
>> (patch merged from https://lore.kernel.org/lkml/20220511060212.GA32192@lst.de/T/)
>>
>> Signed-off-by: Tianyu Lan <Tianyu.Lan at microsoft.com>
>> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
>> ---
>>   kernel/dma/swiotlb.c | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>> index ce241d72ad03..570af75332f5 100644
>> --- a/kernel/dma/swiotlb.c
>> +++ b/kernel/dma/swiotlb.c
>> @@ -766,7 +766,18 @@ dma_addr_t swiotlb_map(struct device *dev, phys_addr_t paddr, size_t size,
>>   
>>   size_t swiotlb_max_mapping_size(struct device *dev)
>>   {
>> -	return ((size_t)IO_TLB_SIZE) * IO_TLB_SEGSIZE;
>> +	int min_align_mask = dma_get_min_align_mask(dev);
>> +	int min_align = 0;
>> +
>> +	/*
>> +	 * swiotlb_find_slots() skips slots according to
>> +	 * min align mask. This affects max mapping size.
>> +	 * Take it into acount here.
>> +	 */
>> +	if (min_align_mask)
>> +		min_align = roundup(min_align_mask, IO_TLB_SIZE);
>> +
>> +	return ((size_t)IO_TLB_SIZE) * IO_TLB_SEGSIZE - min_align;
>>   }
>>   
> 
> I don't like this much, not sure how dma_get_min_align_mask is bound. But given
> this is restricted to linux-azure and the mailing list discussion is leaning
> torwards accepting such a fix:
> 
> Acked-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
> 

I should have mentioned that MSFT has tested this patch. It does seem 
like there are getting to be a lot of swiotlb CVM SAUCE patches. A more 
thorough upstream review would have cut down on the number of tweak patches.

>>   bool is_swiotlb_active(struct device *dev)
>> -- 
>> 2.36.0
>>
>>
>> -- 
>> kernel-team mailing list
>> kernel-team at lists.ubuntu.com
>> https://lists.ubuntu.com/mailman/listinfo/kernel-team

-- 
-----------
Tim Gardner
Canonical, Inc