linux-kernel - RE: [PATCH v2 2/4] iommu/vt-d: Set PGSNP bit in pasid table entry for SVA binding

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Message-ID: <BN9PR11MB5276EDDABE40B6A6A67A0F4E8CF79@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Fri, 22 Apr 2022 03:05:04 +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 2/4] iommu/vt-d: Set PGSNP bit in pasid table entry for
 SVA binding

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Thursday, April 21, 2022 7:36 PM
> 
> This field make the requests snoop processor caches irrespective of
> other attributes in the request or other fields in paging structure
> entries used to translate the request.

I think you want to first point out the fact that SVA wants snoop
cache instead of just talking about the effect of PGSNP.

But thinking more I wonder why PGSNP is ever required. This is
similar to DMA API case. x86 is already cache coherent for normal
DMA (if not setting PCI no-snoop) and if the driver knows no-snoop
is incompatible to SVA API then it should avoid triggering no-snoop
traffic for SVA usage. In this case it is pointless for IOMMU driver
to enable force-snooping. Even in the future certain platform allows
no-snoop usage w/ SVA (I'm not sure how it works) this again should
be reflected by additional SVA APIs for driver to explicitly manage.

force-snoop should be enabled only in device assignment case IMHO,
orthogonal to whether vSVA is actually used.

Did I misunderstand the motivation here?

> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/intel/svm.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
> index 23a38763c1d1..c720d1be992d 100644
> --- a/drivers/iommu/intel/svm.c
> +++ b/drivers/iommu/intel/svm.c
> @@ -391,9 +391,12 @@ static struct iommu_sva
> *intel_svm_bind_mm(struct intel_iommu *iommu,
>  	}
> 
>  	/* Setup the pasid table: */
> -	sflags = (flags & SVM_FLAG_SUPERVISOR_MODE) ?
> -			PASID_FLAG_SUPERVISOR_MODE : 0;
> -	sflags |= cpu_feature_enabled(X86_FEATURE_LA57) ?
> PASID_FLAG_FL5LP : 0;
> +	sflags = PASID_FLAG_PAGE_SNOOP;
> +	if (flags & SVM_FLAG_SUPERVISOR_MODE)
> +		sflags |= PASID_FLAG_SUPERVISOR_MODE;
> +	if (cpu_feature_enabled(X86_FEATURE_LA57))
> +		sflags |= PASID_FLAG_FL5LP;
> +
>  	spin_lock_irqsave(&iommu->lock, iflags);
>  	ret = intel_pasid_setup_first_level(iommu, dev, mm->pgd, mm-
> >pasid,
>  					    FLPT_DEFAULT_DID, sflags);
> --
> 2.25.1