Re: [Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[David Gibson]

On Mon, Jun 19, 2017 at 12:32:48PM +0200, Thomas Huth wrote:
> On 19.06.2017 11:39, Alexander Graf wrote:
> > On 06/15/2017 03:33 PM, Peter Maydell wrote:
> >> I've just noticed that on a SPARC host, some of the PPC guests
> >> warn during make check:
> >>
> >>    /ppc64/prom-env/pseries:
> >> qemu-system-ppc64: System page size 0x2000 is not enabled in
> >> page_size_mask (0x11000). Performance may be slow
> >>
> >> Is this really a performance problem on a TCG guest?
> >> It makes the 'make check' output a bit noisy, but not
> >> unbearably so.
> > 
> > From what I can tell this really only affects DDW which should be page
> > size agnostic when only using emulated devices. Maybe Alexey thought of
> > using VFIO-pci devices in a TCG guest?
> 
> Maybe we could at least silence the warning by checking
> qtest_enabled()?


So,

1) Yes, we should at least silence the warning.

2) Certainly the mentioned slowdown shouldn't be significant for TCG
guests.

3) I _think_ the restriction which causes the slowdown doesn't even
exist for TCG, but I haven't 100% convinced myself of this.  I've been
sick lately, so my brain isn't working all that well.

-- 
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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