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

[Avi Kivity <avi@redhat.com>]

On 05/15/2012 03:38 PM, Christian Borntraeger wrote:
> 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 :-)

Yeah, I even have vague memories of the hack.  Thanks for the
clarifications.

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