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

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Message-ID: <15590835-ed6a-44a5-94b5-663f159653cc@intel.com>
Date: Mon, 24 Jun 2024 16:06:26 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	"Kevin Tian" <kevin.tian@...el.com>, Jacob Pan
	<jacob.jun.pan@...ux.intel.com>
CC: 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

Hi Baolu,

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

> This is a backward compatibility feature which enables older OSs. Since
> Linux VT-d implementation has already supported ATS features for a long
> time, there is no need to rely on this compatibility hardware. Remove it
> to make the driver future-proof.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 9 +--------
>   1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 07e394dfccc1..b63347c8bf5d 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3056,14 +3056,7 @@ static bool dmar_ats_supported(struct pci_dev *dev, struct intel_iommu *iommu)
>   	dev = pci_physfn(dev);
>   	satcu = dmar_find_matched_satc_unit(dev);
>   	if (satcu)
> -		/*
> -		 * This device supports ATS as it is in SATC table.
> -		 * When IOMMU is in legacy mode, enabling ATS is done
> -		 * automatically by HW for the device that requires
> -		 * ATS, hence OS should not enable this device ATS
> -		 * to avoid duplicated TLB invalidation.
> -		 */
> -		return !(satcu->atc_required && !sm_supported(iommu));
> +		return true;
>   
>   	for (bus = dev->bus; bus; bus = bus->parent) {
>   		bridge = bus->self;

-- 
Regards,
Yi Liu