Re: [PATCH 3/3] virtio_balloon: Introduce memory recover

[David Hildenbrand <david@redhat.com>]

On 25.05.22 01:32, zhenwei pi wrote:
> 
> 
> On 5/25/22 03:35, Sean Christopherson wrote:
>> On Fri, May 20, 2022, zhenwei pi wrote:
>>> @@ -59,6 +60,12 @@ enum virtio_balloon_config_read {
>>>   	VIRTIO_BALLOON_CONFIG_READ_CMD_ID = 0,
>>>   };
>>>   
>>> +/* the request body to commucate with host side */
>>> +struct __virtio_balloon_recover {
>>> +	struct virtio_balloon_recover vbr;
>>> +	__virtio32 pfns[VIRTIO_BALLOON_PAGES_PER_PAGE];
>>
>> I assume this is copied from virtio_balloon.pfns, which also uses __virtio32, but
>> isn't that horribly broken?  PFNs are 'unsigned long', i.e. 64 bits on 64-bit kernels.
>> x86-64 at least most definitely generates 64-bit PFNs.  Unless there's magic I'm
>> missing, page_to_balloon_pfn() will truncate PFNs and feed the host bad info.
>>
> 
> Yes, I also noticed this point, I suppose the balloon device can not 
> work on a virtual machine which has physical address larger than 16T.

Yes, that's a historical artifact and we never ran into it in practice
-- because 16TB VMs are still rare, especially when paired with
virtio-balloon inflation/deflation. Most probably the guest should just
stop inflating when hitting such a big PFN. In the future, we might want
a proper sg interface instead.

-- 
Thanks,

David / dhildenb