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

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Message-ID: <e10c2696-23c3-befe-4f4d-25e18918132f@gmail.com>
Date:   Mon, 14 Jun 2021 22:04:06 +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,
        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 6/14/2021 3:09 PM, Christoph Hellwig wrote:
> 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.
> 

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.