Re: [Xen-devel] [PATCHv1] xen/evtchn: dynamically grow pending event channel ring

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/11/15 16:14, David Vrabel wrote:
> If more than 1024 event channels are bound to a evtchn device then it
> possible (even with well behaved applications) for the ring to
> overflow and events to be lost (reported as an -EFBIG error).
>
> Dynamically increase the size of the ring so there is always enough
> space for all bound events.  Well behaved applicables that only unmask
> events after draining them from the ring can thus no longer lose
> events.
>
> However, an application could unmask an event before draining it,
> allowing multiple entries per port to accumulate in the ring, and a
> overflow could still occur.  So the overflow detection and reporting
> is retained.
>
> The ring size is initially only 64 entries so the common use case of
> an application only binding a few events will use less memory than
> before.  The ring size may grow to 512 KiB (enough for all 2^17
> possible channels).  This order 7 kmalloc() may fail due to memory
> fragmentation, so we fall back to trying vmalloc().
>
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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