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    SubjectRe: [PATCH 03/10] x86: add initialization code for DMA-API debugging
    On Fri, Nov 21, 2008 at 06:43:48PM +0100, Ingo Molnar wrote:
    >
    > * Joerg Roedel <joerg.roedel@amd.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.

    > This way if there's irq+DMA traffic on one CPU from one device into
    > one range of memory, and irq+DMA traffic on another CPU to another
    > device, they will map to two different hash buckets.
    >
    > 4)
    >
    > Plus it might be an option to make hash lookup lockless as well:
    > depending on the DMA flux we can get a lot of lookups, and taking the
    > bucket lock can be avoided, if you use RCU-safe list ops and drive the
    > refilling of the free entries pool from RCU.

    Joerg


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