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

[Greg Kurz]

On Fri, 22 Jun 2018 00:01:13 +1000
David Gibson <address@hidden> wrote:

> On Thu, Jun 21, 2018 at 02:24:19PM +0200, Greg Kurz wrote:
> > On Mon, 18 Jun 2018 16:36:05 +1000
> > David Gibson <address@hidden> 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 <address@hidden>
> > > ---
> > >  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.
> 

/me should read more carefully... :(


Reviewed-by: Greg Kurz <address@hidden>

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