Re: [Qemu-devel] [PATCH V3 10/11] vcpu: introduce lockmap

[Jan Kiszka]

On 2012-09-11 13:03, Avi Kivity wrote:
> On 09/11/2012 01:04 PM, Jan Kiszka wrote:
> 
>>> DMA is inherently asynchronous, so we already drop the lock between
>>> initiation and completion; we need to find a way to make it easy to use
>>> the API without taking the lock while the transfer takes place.
>>
>> We will have to review/rework device models that want to use the new
>> locking scheme in a way that they can drop their own lock while issuing
>> DMA. But that is surely non-trivial.
> 
> Most DMA today happens without the big qemu lock.  We only need to
> convert the paths that actually access memory, these are the block and
> network layers (for the respective devices).

...and the devices themselves, of course.

> 
>> The other option is to keep DMA requests issued by devices synchronous
>> but let them fail if we are about to lock up. Still requires changes,
>> but is probably more comprehensible for device model developers.
> 
> How do you handle failures?

By not sending a network frame or dropping an incoming one, e.g., and
signaling this in a device specific way.

Jan

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