Re: [Qemu-devel] [RFC PATCH v0 1/2] spapr: Allocate HTAB from machine init

[David Gibson]

On Thu, Sep 24, 2015 at 09:11:52AM +0530, Bharata B Rao wrote:
> On Wed, Sep 23, 2015 at 01:28:53PM +1000, David Gibson wrote:
> > On Tue, Sep 22, 2015 at 09:09:48AM +0530, Bharata B Rao wrote:
> > > Allocate HTAB from ppc_spapr_init() so that we can abort the guest
> > > if requested HTAB size is't allocated by the host. However retain the
> > > htab reset call in spapr_reset_htab() so that HTAB gets reset (and
> > > not allocated) during machine reset.
> > 
> > I was briefly worried about this, because I recall there as a reason
> > htab allocation got moved to the reset handler in the first place.
> > Looking at the git history, however, I've convinced myself this is
> > basically ok (because you preserve the call during reset to wipe clean
> > the htab).
> > 
> > > Signed-off-by: Bharata B Rao <address@hidden>
> > > ---
> > >  hw/ppc/spapr.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index 7f4f196..4692122 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -979,7 +979,7 @@ static void emulate_spapr_hypercall(PowerPCCPU *cpu)
> > >  #define CLEAN_HPTE(_hpte)  ((*(uint64_t *)(_hpte)) &= 
> > > tswap64(~HPTE64_V_HPTE_DIRTY))
> > >  #define DIRTY_HPTE(_hpte)  ((*(uint64_t *)(_hpte)) |= 
> > > tswap64(HPTE64_V_HPTE_DIRTY))
> > >  
> > > -static void spapr_reset_htab(sPAPRMachineState *spapr)
> > > +static void spapr_alloc_htab(sPAPRMachineState *spapr)
> > >  {
> > >      long shift;
> > >      int index;
> > > @@ -1012,6 +1012,16 @@ static void spapr_reset_htab(sPAPRMachineState 
> > > *spapr)
> > >              DIRTY_HPTE(HPTE(spapr->htab, index));
> > >          }
> > >      }
> > > +}
> > > +
> > > +static void spapr_reset_htab(sPAPRMachineState *spapr)
> > > +{
> > > +    /*
> > > +     * We have already allocated the hash page table, this call will
> > > +     * not again allocate but only result in clearing of hash page
> > > +     * table entries.
> > > +     */
> > > +    kvmppc_reset_htab(spapr->htab_shift);
> > 
> > It's unlikely the kernel will give us less htab than we already have,
> > but we really should at least check for that.  Probably not much we
> > can do except abort() but at least we can give a useful error message.
> 
> With the change I am doing here, this is no longer an allocation path.
> Host kernel will just clear the HTAB and return the same htab_shift
> that we passed here. So do you think it still makes sense to check
> return value ?

That's the current kernel behaviour, but the interface doesn't
guarantee that.  So, yes, I still think you have to check the return
value.

-- 
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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