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

[David Gibson]

On Tue, Mar 26, 2019 at 03:08:20PM +0100, 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)

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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