Re: [qemu-s390x] [PATCH for-4.0?] exec: Only count mapped memory backends for qemu_getrampagesize()

[David Gibson]

On Tue, Mar 26, 2019 at 06:02:51PM +0100, David Hildenbrand wrote:
> On 26.03.19 15:08, Igor Mammedov wrote:
> > On Tue, 26 Mar 2019 14:50:58 +1100
> > David Gibson <address@hidden> wrote:
> > 
> >> qemu_getrampagesize() works out the minimum host page size backing any of
> >> guest RAM.  This is required in a few places, such as for POWER8 PAPR KVM
> >> guests, because limitations of the hardware virtualization mean the guest
> >> can't use pagesizes larger than the host pages backing its memory.
> >>
> >> However, it currently checks against *every* memory backend, whether or not
> >> it is actually mapped into guest memory at the moment.  This is incorrect.
> >>
> >> This can cause a problem attempting to add memory to a POWER8 pseries KVM
> >> guest which is configured to allow hugepages in the guest (e.g.
> >> -machine cap-hpt-max-page-size=16m).  If you attempt to add non-hugepage,
> >> you can (correctly) create a memory backend, however it (correctly) will
> >> throw an error when you attempt to map that memory into the guest by
> >> 'device_add'ing a pc-dimm.
> >>
> >> What's not correct is that if you then reset the guest a startup check
> >> against qemu_getrampagesize() will cause a fatal error because of the new
> >> memory object, even though it's not mapped into the guest.
> > I'd say that backend should be remove by mgmt app since device_add failed
> > instead of leaving it to hang around. (but fatal error either not a nice
> > behavior on QEMU part)
> 
> Indeed, it should be removed. Depending on the options (huge pages with
> prealloc?) memory might be consumed for unused memory. Undesired.

Right, but if the guest initiates a reboot before the management gets
to that, we'll have a crash.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

signature.asc
Description: PGP signature