linux-kernel - Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for netvsc driver

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Message-ID: <7d86307f-83ff-03ad-c6e9-87b455c559b8@gmail.com>
Date:   Tue, 15 Jun 2021 22:31:31 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
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        arnd@...db.de, dave.hansen@...ux.intel.com, luto@...nel.org,
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        hannes@...xchg.org, cai@....pw, krish.sadhukhan@...cle.com,
        saravanand@...com, Tianyu.Lan@...rosoft.com,
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        brijesh.singh@....com, sunilmut@...rosoft.com
Subject: Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for
 netvsc driver

On 6/14/2021 11:33 PM, Christoph Hellwig wrote:
> On Mon, Jun 14, 2021 at 10:04:06PM +0800, Tianyu Lan wrote:
>> The pages in the hv_page_buffer array here are in the kernel linear
>> mapping. The packet sent to host will contain an array which contains
>> transaction data. In the isolation VM, data in the these pages needs to be
>> copied to bounce buffer and so call dma_map_single() here to map these data
>> pages with bounce buffer. The vmbus has ring buffer where the send/receive
>> packets are copied to/from. The ring buffer has been remapped to the extra
>> space above shared gpa boundary/vTom during probing Netvsc driver and so
>> not call dma map function for vmbus ring
>> buffer.
> 
> So why do we have all that PFN magic instead of using struct page or
> the usual kernel I/O buffers that contain a page pointer?
> 

These PFNs originally is part of Hyper-V protocol data and will be sent
to host. Host accepts these GFN and copy data from/to guest memory. The 
translation from va to pa is done by caller that populates the 
hv_page_buffer array. I will try calling dma map function before 
populating struct hv_page_buffer and this can avoid redundant 
translation between PA and VA.