Re: [Qemu-devel] [PATCH 8/9] spapr: Limit available pagesizes to provide a consistent guest environment

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jun 21, 2018 at 02:24:19PM +0200, Greg Kurz wrote:
> On Mon, 18 Jun 2018 16:36:05 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > KVM HV has some limitations (deriving from the hardware) that mean not all
> > host-cpu supported pagesizes may be usable in the guest.  At present this
> > means that KVM guests and TCG guests may see different available page sizes
> > even if they notionally have the same vcpu model.  This is confusing and
> > also prevents migration between TCG and KVM.
> > 
> > This patch makes the environment consistent by always allowing the same set
> > of pagesizes.  Since we can't remove the KVM limitations, we do this by
> > always applying the same limitations it has, even to TCG guests.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  hw/ppc/spapr_caps.c | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> > index 9fc739b3f5..0584c7c6ab 100644
> > --- a/hw/ppc/spapr_caps.c
> > +++ b/hw/ppc/spapr_caps.c
> > @@ -334,6 +334,38 @@ static void cap_hpt_maxpagesize_apply(sPAPRMachineState *spapr,
> >      spapr_check_pagesize(spapr, qemu_getrampagesize(), errp);
> >  }
> >  
> > +static bool spapr_pagesize_cb(void *opaque, uint32_t seg_pshift, uint32_t pshift)
> > +{
> > +    unsigned maxshift = *((unsigned *)opaque);
> > +
> > +    assert(pshift >= seg_pshift);
> > +
> > +    /* Don't allow the guest to use pages bigger than the configured
> > +     * maximum size */
> > +    if (pshift > maxshift) {
> > +        return false;
> > +    }
> > +
> > +    /* For whatever reason, KVM doesn't allow multiple pagesizes
> > +     * within a segment, *except* for the case of 16M pages in a 4k or
> > +     * 64k segment.  Always exclude other cases, so that TCG and KVM
> > +     * guests see a consistent environment */
> 
> Unless I'm missing something, I don't see how we could get "other cases"
> with TCG, at least with the current content of ppc_hash64_opts_POWER7.

You're missing something.  hash64_opts_POWER7 includes 64k pages in a
segment with base page size of 4k.

-- 
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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