netdev - Re: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM support for storvsc driver

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Date:   Sat, 21 Aug 2021 02:04:11 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
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Subject: Re: [PATCH V3 13/13] HV/Storvsc: Add Isolation VM support for storvsc
 driver



On 8/21/2021 12:08 AM, Michael Kelley wrote:
>>>>    	}
>>> The whole approach here is to do dma remapping on each individual page
>>> of the I/O buffer.  But wouldn't it be possible to use dma_map_sg() to map
>>> each scatterlist entry as a unit?  Each scatterlist entry describes a range of
>>> physically contiguous memory.  After dma_map_sg(), the resulting dma
>>> address must also refer to a physically contiguous range in the swiotlb
>>> bounce buffer memory.   So at the top of the "for" loop over the scatterlist
>>> entries, do dma_map_sg() if we're in an isolated VM.  Then compute the
>>> hvpfn value based on the dma address instead of sg_page().  But everything
>>> else is the same, and the inner loop for populating the pfn_arry is unmodified.
>>> Furthermore, the dma_range array that you've added is not needed, since
>>> scatterlist entries already have a dma_address field for saving the mapped
>>> address, and dma_unmap_sg() uses that field.
>> I don't use dma_map_sg() here in order to avoid introducing one more
>> loop(e,g dma_map_sg()). We already have a loop to populate
>> cmd_request->dma_range[] and so do the dma map in the same loop.
>>
> I'm not seeing where the additional loop comes from.  Storvsc
> already has a loop through the sgl entries.  Retain that loop and call
> dma_map_sg() with nents set to 1.  Then the sequence is
> dma_map_sg() --> dma_map_sg_attrs() --> dma_direct_map_sg() ->
> dma_direct_map_page().  The latter function will call swiotlb_map()
> to map all pages of the sgl entry as a single operation.

After dma_map_sg(), we still need to go through scatter list again to 
populate payload->rrange.pfn_array. We may just go through the scatter 
list just once if dma_map_sg() accepts a callback and run it during go
through scatter list.