linux-kernel - Re: [PATCH v3 1/2] iommu/vt-d: Add helper to flush caches for context change

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Message-ID: <20240702085749.2e2bbea5@jacob-builder>
Date: Tue, 2 Jul 2024 08:57:49 -0700
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, Robin
 Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>, Kevin Tian
 <kevin.tian@...el.com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v3 1/2] iommu/vt-d: Add helper to flush caches for
 context change


On Tue, 2 Jul 2024 12:43:41 +0800, Baolu Lu <baolu.lu@...ux.intel.com>
wrote:

> On 2024/7/2 12:41, Jacob Pan wrote:
> > On Mon,  1 Jul 2024 19:23:16 +0800, Lu Baolu<baolu.lu@...ux.intel.com>
> > wrote:
> >   
> >> +	if (flush_domains) {
> >> +		/*
> >> +		 * If the IOMMU is running in scalable mode and there
> >> might
> >> +		 * be potential PASID translations, the caller should
> >> hold
> >> +		 * the lock to ensure that context changes and cache
> >> flushes
> >> +		 * are atomic.
> >> +		 */
> >> +		assert_spin_locked(&iommu->lock);
> >> +		for (i = 0; i < info->pasid_table->max_pasid; i++) {
> >> +			pte = intel_pasid_get_entry(info->dev, i);
> >> +			if (!pte || !pasid_pte_is_present(pte))
> >> +				continue;  
> > Is it worth going through 1M PASIDs just to skip the PASID cache
> > invalidation? Or just do the flush on all used DIDs unconditionally.  
> 
> Currently we don't track all domains attached to a device. If such
> optimization is necessary, perhaps we can add it later.

I think it is necessary, because without tracking domain IDs, the code
above would have duplicated invalidations.
For example: a device PASID table has the following entries
	PASID	DomainID
-------------------------
	100	1
	200	1
	300	2
-------------------------
When a present context entry changes, we need to do:
qi_flush_pasid_cache(iommu, 1, QI_PC_ALL_PASIDS, 0);
qi_flush_pasid_cache(iommu, 2, QI_PC_ALL_PASIDS, 0);

With this code, we do
qi_flush_pasid_cache(iommu, 1, QI_PC_ALL_PASIDS, 0);
qi_flush_pasid_cache(iommu, 1, QI_PC_ALL_PASIDS, 0);//duplicated!
qi_flush_pasid_cache(iommu, 2, QI_PC_ALL_PASIDS, 0);

Thanks,

Jacob