[SeaBIOS] [Qemu-devel] [RFC PATCH v3 05/19] Implement dimm device abstraction

[Stefan Hajnoczi]

On Wed, Oct 31, 2012 at 12:15 PM, Avi Kivity <avi at redhat.com> wrote:
> On 10/24/2012 10:06 AM, liu ping fan wrote:
>> On Tue, Oct 23, 2012 at 8:25 PM, Stefan Hajnoczi <stefanha at gmail.com> wrote:
>>> On Fri, Sep 21, 2012 at 01:17:21PM +0200, Vasilis Liaskovitis wrote:
>>>> +static void dimm_populate(DimmDevice *s)
>>>> +{
>>>> +    DeviceState *dev= (DeviceState*)s;
>>>> +    MemoryRegion *new = NULL;
>>>> +
>>>> +    new = g_malloc(sizeof(MemoryRegion));
>>>> +    memory_region_init_ram(new, dev->id, s->size);
>>>> +    vmstate_register_ram_global(new);
>>>> +    memory_region_add_subregion(get_system_memory(), s->start, new);
>>>> +    s->mr = new;
>>>> +}
>>>> +
>>>> +static void dimm_depopulate(DimmDevice *s)
>>>> +{
>>>> +    assert(s);
>>>> +    vmstate_unregister_ram(s->mr, NULL);
>>>> +    memory_region_del_subregion(get_system_memory(), s->mr);
>>>> +    memory_region_destroy(s->mr);
>>>> +    s->mr = NULL;
>>>> +}
>>>
>>> How is dimm hot unplug protected against callers who currently have RAM
>>> mapped (from cpu_physical_memory_map())?
>>>
>>> Emulated devices call cpu_physical_memory_map() directly or indirectly
>>> through DMA emulation code.  The RAM pointer may be held for arbitrary
>>> lengths of time, across main loop iterations, etc.
>>>
>>> It's not clear to me that it is safe to unplug a DIMM that has network
>>> or disk I/O buffers, for example.  We also need to be robust against
>>> malicious guests who abuse the hotplug lifecycle.  QEMU should never be
>>> left with dangling pointers.
>>>
>> Not sure about the block layer. But I think those thread are already
>> out of big lock, so there should be a MemoryListener to catch the
>> RAM-unplug event, and if needed, bdrv_flush.
>
>
> IMO we should use the same mechanism as proposed for other devices:
> address_space_map() should grab a reference on the dimm device, and
> address_space_unmap() can release it.  This way device destruction will
> be deferred as soon as all devices complete I/O.
>
> We will have to be careful with network receive buffers though, since
> they can be held indefinitely.

Network receive buffers aren't mapped.  Net receive is not zero-copy.
For example, virtio-net does virtqueue_pop() inside
virtio_net_receive().

I don't see a problem with networking.

Stefan