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

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Date:   Mon, 1 Mar 2021 19:45:36 +0000
From:   Sunil Muthuswamy <sunilmut@...rosoft.com>
To:     Tianyu Lan <ltykernel@...il.com>,
        Christoph Hellwig <hch@...radead.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "brijesh.singh@....com" <brijesh.singh@....com>
Subject: RE: [EXTERNAL] 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.
What is blocking us from making the Hyper-V drivers use the DMA API's? They
will be a null-op generally, when there is no bounce buffer support needed.

>       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.
How does this make a difference here?

>       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.
> 
There are alternative implementations of swiotlb on top of the core swiotlb
API's. One option is to have Hyper-V specific swiotlb wrapper DMA API's with
the custom logic above.

> 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.
I agree with Christoph's comment that in principle, this should be handled using
the DMA API's