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

[David Gibson]

On Wed, Mar 27, 2019 at 03:22:58PM +0100, David Hildenbrand wrote:
> On 27.03.19 10:03, David Gibson wrote:
> > On Wed, Mar 27, 2019 at 09:10:01AM +0100, David Hildenbrand wrote:
> >> On 27.03.19 01:12, David Gibson wrote:
> >>> 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.
> >>>
> >>
> >> Yes, I agree.
> >>
> >> At least on s390x (extending on what Igor said):
> >>
> >> mc->init() -> s390_memory_init() ->
> >> memory_region_allocate_system_memory() -> host_memory_backend_set_mapped()
> >>
> >>
> >> ac->init_machine() -> kvm_arch_init() ->
> >> kvm_s390_configure_mempath_backing() -> qemu_getrampagesize()
> >>
> >>
> >> And in vl.c
> >>
> >> configure_accelerator(current_machine, argv[0]);
> >> ...
> >> machine_run_board_init()
> >>
> >> So memory is indeed not mapped before calling qemu_getrampagesize().
> >>
> >>
> >> We *could* move the call to kvm_s390_configure_mempath_backing() to
> >> s390_memory_init().
> >>
> >> cap_hpage_1m is not needed before we create VCPUs, so this would work fine.
> >>
> >> We could than eventually make qemu_getrampagesize() asssert if no
> >> backends are mapped at all, to catch other user that rely on this being
> >> correct.
> > 
> > So.. I had a look at the usage in kvm_s390_configure_mempath_backing()
> > and I'm pretty sure it's broken.  It will work in the case where
> > there's only one backend.  And if that's the default -mem-path rather
> > than an explicit memory backend then my patch won't break it any
> > further.
> 
> On the second look, I think I get your point.
> 
> 1. Why on earth does "find_max_supported_pagesize" find the "minimum
> page size". What kind of nasty stuff is this.

Ah, yeah, the naming is bad because of history.

The original usecase of this was because on POWER (before POWER9) the
way MMU virtualization works, pages inserted into the guest MMU view
have to be host-contiguous: there's no 2nd level translation that lets
them be broken into smaller host pages.

The upshot is that a KVM guest can only use large pages if it's backed
by large pages on the host.  We have to advertise the availability of
large pages to the guest at boot time though, and there's no way to
restrict it to certain parts of guest RAM.

So, this code path was finding the _maximum_ page size the guest could
use... which depends on the _minimum page_ size used on the host.
When this was moved to (partly) generic code we didn't think to
improve all the names.

> 2. qemu_mempath_getpagesize() is not affected by your patch

Correct.

> and that
> seems to be the only thing used on s390x for now.

Uh.. what?

> I sent a patch to move the call on s390x. But we really have to detect
> the maximum page size (what find_max_supported_pagesize promises), not
> the minimum page size.

Well.. sort of.  In the ppc case it really is the minimum page size we
care about, in the sense that if some part of RAM has a larger page
size, that's fine - even if it's a weird size that we didn't expect.

IIUC for s390 the problem is that KVM doesn't necessarily support
putting large pages into the guest at all, and what large page sizes
it can put into the guest depends on the KVM version.

Finding the maximum backing page size answers that question only on
the assumption that a KVM which supports page size X will support all
smaller page sizes.  I imagine that's true in practice, but does it
have to be true in theory?  If not, it seems safer to me to explicitly
step through every (mapped) backend and individually check if it has a
supported pagesize, rather than only testing the max page size.

It also occurs to me: why does this logic need to be in qemu at all?
KVM must know what pagesizes it supports, and I assume it will throw
an error if you try to put something with the wrong size into a
memslot.  So can't qemu just report that error, rather than checking
the pagesizes itself?

> > qemu_getrampagesize() returns the smallest host page size for any
> > memory backend.  That's what matters for pcc KVM (in several cases)
> > because we need certain things to be host-contiguous, not just
> > guest-contiguous.  Bigger host page sizes are fine for that purpose,
> > clearly.
> > 
> > AFAICT on s390 you're looking to determine if any backend is using
> > hugepages, because KVM may not support that.  The minimum host page
> > size isn't adequate to determine that, so qemu_getrampagesize() won't
> > tell you what you need.
> 
> Indeed.

-- 
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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