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Date:   Mon, 14 Jun 2021 09:09:03 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Tianyu Lan <ltykernel@gmail.com>
Cc:     Christoph Hellwig <hch@lst.de>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com, wei.liu@kernel.org,
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        krish.sadhukhan@oracle.com, saravanand@fb.com,
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Subject: Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for
 netvsc driver
Message-ID: <20210614070903.GA29976@lst.de>
References: <20210530150628.2063957-1-ltykernel@gmail.com> <20210530150628.2063957-11-ltykernel@gmail.com> <20210607065007.GE24478@lst.de> <279cb4bf-c5b6-6db9-0f1e-9238e902c8f2@gmail.com>
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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.