linux-kernel - Re: [PATCH 2/2] iommu/vt-d: Remove hardware automatic ATS dependency

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Message-ID: <ba6b6baf-6fe7-4238-91ee-262ac006c25f@linux.intel.com>
Date: Mon, 24 Jun 2024 16:11:24 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Yi Liu <yi.l.liu@...el.com>, Joerg Roedel <joro@...tes.org>,
 Kevin Tian <kevin.tian@...el.com>, Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc: baolu.lu@...ux.intel.com, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu/vt-d: Remove hardware automatic ATS dependency

On 2024/6/24 16:06, Yi Liu wrote:
> On 2024/6/24 13:25, Lu Baolu wrote:
>> If a device is listed in the SATC table with ATC_REQUIRED flag set, it
>> indicates that the device has a functional requirement to enable its ATC
>> (via the ATS capability) for device operation. However, when IOMMU is
>> running in the legacy mode, ATS could be automatically supported by the
>> hardware so that the OS has no need to support the ATS functionality.
> 
> It seems like if VT-d hw is operating in legacy mode, ATS is enabled
> automatically? 

Yes.

> This is really suprising. Maybe I got it wrongly. Could you
> elaborate a bit?

It's for backward compatible purpose, but I have no hardware details.

Best regards,
baolu