linux-kernel - Re: [PATCH v3 1/4] iommu/vt-d: Implement domain ops for attach_dev

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Message-ID: <20220519140543.1b43ef85@jacob-builder>
Date:   Thu, 19 May 2022 14:05:43 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>, dmaengine@...r.kernel.org,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>, vkoul@...nel.org,
        robin.murphy@....com, will@...nel.org, Yi Liu <yi.l.liu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Eric Auger <eric.auger@...hat.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v3 1/4] iommu/vt-d: Implement domain ops for
 attach_dev_pasid

Hi Jason,

On Wed, 18 May 2022 15:52:05 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:

> On Wed, May 18, 2022 at 11:42:04AM -0700, Jacob Pan wrote:
> 
> > > Yes.. It seems inefficient to iterate over that xarray multiple times
> > > on the flush hot path, but maybe there is little choice. Try to use
> > > use the xas iterators under the xa_lock spinlock..
> > >   
> > xas_for_each takes a max range, here we don't really have one. So I
> > posted v4 w/o using the xas advanced API. Please let me know if you have
> > suggestions.  
> 
> You are supposed to use ULONG_MAX in cases like that.
> 
got it.
> > xa_for_each takes RCU read lock, it should be fast for tlb flush,
> > right? The worst case maybe over flush when we have stale data but
> > should be very rare.  
> 
> Not really, xa_for_each walks the tree for every iteration, it is
> slower than a linked list walk in any cases where the xarray is
> multi-node. xas_for_each is able to retain a pointer where it is in
> the tree so each iteration is usually just a pointer increment.
> 
Thanks for explaining, yeah if we have to iterate multiple times
xas_for_each() is better.

> The downside is you cannot sleep while doing xas_for_each
> 
will do under RCU read lock

> > > The challenge will be accessing the group xa in the first place, but
> > > maybe the core code can gain a function call to return a pointer to
> > > that XA or something..  
>  
> > I added a helper function to find the matching DMA API PASID in v4.  
> 
> Again, why are we focused on DMA API? Nothing you build here should be
> DMA API beyond the fact that the iommu_domain being attached is the
> default domain.
The helper is not DMA API specific. Just a domain-PASID look up. Sorry for
the confusion.

Thanks,

Jacob