Re: [PATCH 1/4] kvm-s390: add capability indicating COW support

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 15/05/12 14:33, Avi Kivity wrote:
> On 05/15/2012 03:31 PM, Christian Borntraeger wrote:
>> On 15/05/12 14:24, Avi Kivity wrote:
>>>> Newer systems allow to write-protect the guest backing memory
>>>> and let the fault be delivered to the host, thus allowing COW.
>>>>
>>>> Use a capability bit to tell qemu if that is possible.
>>>>
>>>
>>> Asking out of ignorance: who is doing the write protection here? The
>>> guest?  If so, why is qemu involved?
>>
>> It is about the host doing write protection of guest/user memory (e.g. for 
>> dirty pages tracking or KSM)
> 
> Ok, so why does qemu^Wuserspace need to know about it?

By default qemu will use MAP_PRIVATE for guest pages. This will write
protect pages and thus break on s390 systems that dont support this feature.
Therefore qemu has a hack to always use MAP_SHARED for s390. But MAP_SHARED
has other problems (no dirty pages tracking, a lot more swap overhead etc.)
With this feature qemu can use the standard qemu alloc if available, otherwise
it will use the old s390 hack. I will put you on cc for the qemu patch :-)

Christian