linux-kernel - Re: [PATCH hyperv-next 6/6] drivers: SCSI: Do not bounce-bufffer for the confidential VMBus

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Message-ID: <d8876120-5478-4d2f-acad-b0a59261bbc5@linux.microsoft.com>
Date: Wed, 9 Apr 2025 08:36:43 -0700
From: Roman Kisel <romank@...ux.microsoft.com>
To: Christoph Hellwig <hch@....de>
Cc: aleksander.lobakin@...el.com, andriy.shevchenko@...ux.intel.com,
 arnd@...db.de, bp@...en8.de, catalin.marinas@....com, corbet@....net,
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 linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
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 sunilmut@...rosoft.com
Subject: Re: [PATCH hyperv-next 6/6] drivers: SCSI: Do not bounce-bufffer for
 the confidential VMBus



On 4/9/2025 3:53 AM, Christoph Hellwig wrote:
> On Tue, Apr 08, 2025 at 05:08:35PM -0700, Roman Kisel wrote:
>> The device bit that indicates that the device is capable of I/O
>> with private pages lets avoid excessive copying in the Hyper-V
>> SCSI driver.
>>
>> Set that bit equal to the confidential external memory one to
>> not bounce buffer
> 
> Drivers have absolutely no business telling this.  The need for bounce
> buffering or not is a platform/IOMMU decision and not one specific to
> a certain device or driver.

Seemed to work although I cannot claim nothing is going to be broken
ever. It did appear from the code that one could have this per-device
bit.

As I understand, you're saying this is architecturally broken. Do you
think a broader set of changes would improve the implementation?

> 

-- 
Thank you,
Roman