Re: [Qemu-devel] [patch v5 5/8] memory: introduce local lock for address space

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-11-01 16:45, Avi Kivity wrote:
> On 10/29/2012 11:46 AM, liu ping fan wrote:
>> On Mon, Oct 29, 2012 at 5:32 PM, Avi Kivity <avi@redhat.com> wrote:
>>> On 10/29/2012 01:48 AM, Liu Ping Fan wrote:
>>>> For those address spaces which want to be able out of big lock, they
>>>> will be protected by their own local.
>>>>
>>>> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>>>> ---
>>>>  memory.c |   11 ++++++++++-
>>>>  memory.h |    5 ++++-
>>>>  2 files changed, 14 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/memory.c b/memory.c
>>>> index 2f68d67..ff34aed 100644
>>>> --- a/memory.c
>>>> +++ b/memory.c
>>>> @@ -1532,9 +1532,15 @@ void memory_listener_unregister(MemoryListener *listener)
>>>>      QTAILQ_REMOVE(&memory_listeners, listener, link);
>>>>  }
>>>>
>>>> -void address_space_init(AddressSpace *as, MemoryRegion *root)
>>>> +void address_space_init(AddressSpace *as, MemoryRegion *root, bool lock)
>>>
>>>
>>> Why not always use the lock?  Even if the big lock is taken, it doesn't
>>> hurt.  And eventually all address spaces will be fine-grained.
>>>
>> I had thought only mmio is out of big lock's protection. While others
>> address space will take extra expense. So leave them until they are
>> ready to be out of big lock.
> 
> The other address spaces are pio (which also needs fine-grained locking)
> and the dma address spaces (which are like address_space_memory, except
> they are accessed via DMA instead of from the vcpu).

The problem is with memory regions that don't do fine-grained locking
yet, thus don't provide ref/unref. Then we fall back to taking BQL
across dispatch. If the dispatch caller already holds the BQL, we will
bail out.

As I understand the series, as->lock == NULL means that we will never
take any lock during dispatch as the caller is not yet ready for
fine-grained locking. This prevents the problem - for PIO at least. But
this series should break TCG as it calls into MMIO dispatch from the
VCPU while holding the BQL.

Jan



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