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From:   Christoph Hellwig <hch@lst.de>
To:     Tianyu Lan <ltykernel@gmail.com>
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Subject: Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for
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On Mon, Jun 07, 2021 at 11:21:20PM +0800, Tianyu Lan wrote:
>> dma_map_single can only be used on page baked memory, and if this is
>> using page backed memory you wouldn't need to do thee phys_to_virt
>> tricks.  Can someone explain the mess here in more detail?
>
> Sorry. Could you elaborate the issue? These pages in the pb array are not 
> allocated by DMA API and using dma_map_single() here is to map these pages' 
> address to bounce buffer physical address.

dma_map_single just calls dma_map_page using virt_to_page.  So this
can't work on addresses not in the kernel linear mapping.