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

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Date:   Mon, 14 Jun 2021 17:33:39 +0200
From:   Christoph Hellwig <hch@....de>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     Christoph Hellwig <hch@....de>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, arnd@...db.de,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        rppt@...nel.org, hannes@...xchg.org, cai@....pw,
        krish.sadhukhan@...cle.com, saravanand@...com,
        Tianyu.Lan@...rosoft.com, konrad.wilk@...cle.com,
        m.szyprowski@...sung.com, robin.murphy@....com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, joro@...tes.org, will@...nel.org,
        xen-devel@...ts.xenproject.org, davem@...emloft.net,
        kuba@...nel.org, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        iommu@...ts.linux-foundation.org, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, netdev@...r.kernel.org,
        vkuznets@...hat.com, thomas.lendacky@....com,
        brijesh.singh@....com, sunilmut@...rosoft.com
Subject: Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for
 netvsc driver

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?