linux-kernel - Re: [PATCH] iommu/intel-iommu: fix pasid table size encoding

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Date:   Tue, 6 Dec 2016 17:31:11 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Mika Kuoppala <mika.kuoppala@...ux.intel.com>
Subject: Re: [PATCH] iommu/intel-iommu: fix pasid table size encoding

Hi Jacob,

On Thu, Dec 01, 2016 at 01:50:26PM -0800, Jacob Pan wrote:
> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> index 27596e6..f112aa9 100644
> --- a/drivers/iommu/intel-iommu.c
> +++ b/drivers/iommu/intel-iommu.c
> @@ -5173,6 +5173,29 @@ static void intel_iommu_remove_device(struct device *dev)
>  }
>  
>  #ifdef CONFIG_INTEL_IOMMU_SVM
> +#define MAX_NR_PASID_BITS (20)
> +static inline unsigned long intel_iommu_get_pts(struct intel_iommu *iommu)
> +{
> +	unsigned long pts;
> +
> +	/*
> +	 * Convert ecap_pss to extend context entry pts encoding, also
> +	 * respect the soft pasid_max value set by the iommu.
> +	 * - number of PASID bits = ecap_pss + 1
> +	 * - number of PASID table entries = 2^(pts + 5)
> +	 * Therefore, pts = ecap_pss - 4
> +	 * e.g. KBL ecap_pss = 0x13, PASID has 20 bits, pts = 15
> +	 */
> +	if (ecap_pss(iommu->ecap) < 5)
> +		return 0;
> +
> +	pts = (ecap_pss(iommu->ecap) - 4);
> +
> +	/* pasid_max is encoded as actual number of entries not the bits */
> +	return min(find_first_bit((unsigned long *)&iommu->pasid_max,
> +					MAX_NR_PASID_BITS) - 5, pts);

Iommu->max_pasid already depends on ecap_pss(), so I think it is
better to just calculate the pts value as ffs(iommu->max_pasid) - 5.
This way you don't need an extra function and have a simpler
calculation.


	Joerg