On Tue, Mar 26, 2019 at 03:08:20PM +0100, Igor Mammedov wrote: > On Tue, 26 Mar 2019 14:50:58 +1100 > David Gibson <da...@gibson.dropbear.id.au> 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)
I agree, but reset could be guest initiated, so even if management is
doing the right thing there's a window where it could break.
> > This patch corrects the problem by adjusting find_max_supported_pagesize()
> > (called from qemu_getrampagesize() via object_child_foreach) to exclude
> > non-mapped memory backends.
> I'm not sure about if it's ok do so. It depends on where from
> qemu_getrampagesize() is called. For example s390 calls it rather early
> when initializing KVM, so there isn't anything mapped yet.
Oh. Drat.
> And once I replace -mem-path with hostmem backend and drop
> qemu_mempath_getpagesize(mem_path) /which btw aren't guarantied to be mapped
> either/
> this patch might lead to incorrect results for initial memory as
> well.
Uh.. I don't immediately see why.
--
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
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