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Date:   Mon, 2 Aug 2021 22:08:43 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
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        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
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        martin.petersen@...cle.com, arnd@...db.de, hch@....de,
        m.szyprowski@...sung.com, robin.murphy@....com,
        thomas.lendacky@....com, brijesh.singh@....com, ardb@...nel.org,
        Tianyu.Lan@...rosoft.com, rientjes@...gle.com,
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        linux-arch@...r.kernel.org, linux-hyperv@...r.kernel.org,
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Subject: Re: [PATCH 13/13] HV/Storvsc: Add Isolation VM support for storvsc
 driver

On 8/2/2021 9:20 PM, Joerg Roedel wrote:
> On Wed, Jul 28, 2021 at 10:52:28AM -0400, Tianyu Lan wrote:
>> In Isolation VM, all shared memory with host needs to mark visible
>> to host via hvcall. vmbus_establish_gpadl() has already done it for
>> storvsc rx/tx ring buffer. The page buffer used by vmbus_sendpacket_
>> mpb_desc() still need to handle. Use DMA API to map/umap these
>> memory during sending/receiving packet and Hyper-V DMA ops callback
>> will use swiotlb function to allocate bounce buffer and copy data
>> from/to bounce buffer.
> 
> I am wondering why you dont't use DMA-API unconditionally? It provides
> enough abstraction to do the right thing for isolated and legacy VMs.
> 

In VMbus, there is already a similar bounce buffer design and so there 
is no need to call DMA-API for such buffer. Calling DMA-API is to use
swiotlb bounce buffer for those buffer which hasn't been covered. This
is why need to conditionally call DMA API.

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