linux-kernel - Re: [RFC PATCH 12/12] HV/Storvsc: Add bounce buffer support for Storvsc

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Date:   Mon, 1 Mar 2021 21:43:45 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        thomas.lendacky@....com, brijesh.singh@....com,
        sunilmut@...rosoft.com
Subject: Re: [RFC PATCH 12/12] HV/Storvsc: Add bounce buffer support for
 Storvsc

Hi Christoph:
      Thanks a lot for your review. There are some reasons.
      1) Vmbus drivers don't use DMA API now.
      2) Hyper-V Vmbus channel ring buffer already play bounce buffer 
role for most vmbus drivers. Just two kinds of packets from 
netvsc/storvsc are uncovered.
      3) In AMD SEV-SNP based Hyper-V guest, the access physical address 
of shared memory should be bounce buffer memory physical address plus
with a shared memory boundary(e.g, 48bit) reported Hyper-V CPUID. It's
called virtual top of memory(vTom) in AMD spec and works as a watermark. 
So it needs to ioremap/memremap the associated physical address above 
the share memory boundary before accessing them. swiotlb_bounce() uses
low end physical address to access bounce buffer and this doesn't work 
in this senario. If something wrong, please help me correct me.

Thanks.


On 3/1/2021 2:54 PM, Christoph Hellwig wrote:
> This should be handled by the DMA mapping layer, just like for native
> SEV support.
>