linux-kernel - Re: [PATCH 03/10] x86: add initialization code for DMA-API debugging

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Message-ID: <20081123113543.GD29663@elte.hu>
Date:	Sun, 23 Nov 2008 12:35:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Joerg Roedel <joerg.roedel@....com>, netdev@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 03/10] x86: add initialization code for DMA-API
	debugging


* Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Joerg Roedel <joro@...tes.org> wrote:
> 
> > On Fri, Nov 21, 2008 at 06:43:48PM +0100, Ingo Molnar wrote:
> > > 
> > > * Joerg Roedel <joerg.roedel@....com> wrote:
> > > 
> > > > +static struct list_head dma_entry_hash[HASH_SIZE];
> > > > +
> > > > +/* A slab cache to allocate dma_map_entries fast */
> > > > +static struct kmem_cache *dma_entry_cache;
> > > > +
> > > > +/* lock to protect the data structures */
> > > > +static DEFINE_SPINLOCK(dma_lock);
> > > 
> > > some more generic comments about the data structure: it's main purpose 
> > > is to provide a mapping based on (dev,addr). There's little if any 
> > > cross-entry interaction - same-address+same-dev DMA is checked.
> > > 
> > > 1)
> > > 
> > > the hash:
> > > 
> > > + 	return (entry->dev_addr >> HASH_FN_SHIFT) & HASH_FN_MASK;
> > > 
> > > should mix in entry->dev as well - that way we get not just per 
> > > address but per device hash space separation as well.
> > > 
> > > 2)
> > > 
> > > HASH_FN_SHIFT is 1MB chunks right now - that's probably fine in 
> > > practice albeit perhaps a bit too small. There's seldom any coherency 
> > > between the physical addresses of DMA - we rarely have any real 
> > > (performance-relevant) physical co-location of DMA addresses beyond 4K 
> > > granularity. So using 1MB chunking here will discard a good deal of 
> > > random low bits we should be hashing on.
> > > 
> > > 3)
> > > 
> > > And the most scalable locking would be per hash bucket locking - no 
> > > global lock is needed. The bucket hash heads should probably be 
> > > cacheline sized - so we'd get one lock per bucket.
> > 
> > Hmm, I just had the idea of saving this data in struct device. How 
> > about that? The locking should scale too and we can extend it 
> > easier. For example it simplifys a per-device disable function for 
> > the checking. Or another future feature might be leak tracing.
> 
> that will help with spreading the hash across devices, but brings in 
> lifetime issues: you must be absolutely sure all DMA has drained at 
> the point a device is deinitialized.

Note that obviously proper DMA quiescence is a must-have during device 
dinit anyway, but still, it's an extra complication to init/deinit the 
hashes, etc.

	Ingo
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