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Re: [Qemu-devel] [PATCH v7 0/7] push mmio dispatch out of big lock

From: Jan Kiszka
Subject: Re: [Qemu-devel] [PATCH v7 0/7] push mmio dispatch out of big lock
Date: Mon, 06 May 2013 13:39:32 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

On 2013-05-06 13:28, Paolo Bonzini wrote:
> Il 06/05/2013 13:11, Jan Kiszka ha scritto:
>> On 2013-05-06 12:58, Paolo Bonzini wrote:
>>> Il 06/05/2013 12:56, Jan Kiszka ha scritto:
>>>>> The problem is that even if I/O for a region is supposed to happen
>>>>> within the BQL, lookup can happen outside the BQL.  Lookup will use the
>>>>> region even if it is just to discard it:
>>>>>            VCPU thread (under BQL)              device thread
>>>>> --------------------------------------------------------------------------------------
>>>>>                                                 flatview_ref
>>>>>                                                 memory_region_find 
>>>>> returns d->mr
>>>>>                                                 memory_region_ref(d->mr) 
>>>>> /* nop */
>>>>>            qdev_free(d)
>>>>>              object_unparent(d)
>>>>>                unrealize(d)
>>>>>                  memory_region_del_subregion(d->mr)
>>>>>                    FlatView updated, d->mr not in the new view
>>>>>                                                 flatview_unref
>>>>> memory_region_unref(d->mr)
>>>>>                                                     object_unref(d)
>>>>>                                                       free(d)
>>>>>                                                 if (!d->mr->is_ram) {     
>>>>>    /* BAD! */
>>>>> memory_region_unref(d->mr) /* nop */
>>>>>                                                   return error
>>>>>                                                 }
>>>>> Here, the memory region is dereferenced *before* we know that it is 
>>>>> BQL-free
>>>>> (in fact, exactly to ascertain whether it is BQL-free).
>>>> Both flatview update and lookup *plus* locking type evaluation (i.e.
>>>> memory region dereferencing) always happen under the address space lock.
>>>> See Pingfan's patch.
>>> That's true of address_space_rw/map, but I don't think it holds for
>>> memory_region_find.
>> It has to, or it would be broken: Either it is called on a region that
>> supports reference counting
> You cannot know that in advance, can you?  The address is decided by the
> guest.

Need to help me again to get the context: In which case is this a
hot-path that we want to keep BQL-free? Current users of
memory_region_find appear to be all relatively slow paths, thus are fine
with staying under BQL.

>> and, thus, increments the counter before
>> returning, or it has to be called with the BQL held.
> ... or we need to support reference counting on all regions, so that the
> other possibility is automatically true.
> Strictly speaking, only regions that can be unplugged need to support
> reference counting.

That should make the conversion, if actually required, more bearable.
Having to assign an owner to every region around as a precondition to
introduce a new concept with initially less than a handful of users
would be too much, I suppose.


Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux

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