linux-kernel - RE: [PATCH v2 4/4] iommu/vt-d: Size Page Request Queue to avoid overflow condition

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Message-ID: <BN9PR11MB5276F593B25A1D3C8B2595638CF79@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Fri, 22 Apr 2022 03:07:05 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 4/4] iommu/vt-d: Size Page Request Queue to avoid
 overflow condition

> From: Lu Baolu
> Sent: Thursday, April 21, 2022 7:36 PM
> 
> PRQ overflow may cause I/O throughput congestion, resulting in unnecessary
> degradation of I/O performance. Appropriately increasing the length of PRQ
> can greatly reduce the occurrence of PRQ overflow. The count of maximum
> page requests that can be generated in parallel by a PCIe device is
> statically defined in the Outstanding Page Request Capacity field of the
> PCIe ATS configure space.
> 
> The new length of PRQ is calculated by summing up the value of Outstanding
> Page Request Capacity register across all devices where Page Requests are
> supported on the real PR-capable platform (Intel Sapphire Rapids). The
> result is round to the nearest higher power of 2.
> 
> The PRQ length is also double sized as the VT-d IOMMU driver only updates
> the Page Request Queue Head Register (PQH_REG) after processing the
> entire
> queue.
> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>

> ---
>  include/linux/intel-svm.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/intel-svm.h b/include/linux/intel-svm.h
> index b3b125b332aa..207ef06ba3e1 100644
> --- a/include/linux/intel-svm.h
> +++ b/include/linux/intel-svm.h
> @@ -9,7 +9,7 @@
>  #define __INTEL_SVM_H__
> 
>  /* Page Request Queue depth */
> -#define PRQ_ORDER	2
> +#define PRQ_ORDER	4
>  #define PRQ_RING_MASK	((0x1000 << PRQ_ORDER) - 0x20)
>  #define PRQ_DEPTH	((0x1000 << PRQ_ORDER) >> 5)
> 
> --
> 2.25.1
> 
> _______________________________________________
> iommu mailing list
> iommu@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

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