Re: [Qemu-devel] [RFC PATCH v2 08/12] spapr: Only setup HTP if necessary.

[Suraj Jitindar Singh <sjitindarsingh@gmail.com>]

On Tue, 2017-02-28 at 11:28 +1100, David Gibson wrote:
> s/HTP/HPT/ in subject line.
> 
> 
> On Thu, Feb 23, 2017 at 05:00:01PM +1100, Sam Bobroff wrote:
> > 
> > If QEMU is using KVM, and KVM is capable of running in radix mode,
> > guests can be run in real-mode without allocating a HPT (because
> > KVM
> > will use a minimal RPT). So in this case, we avoid creating the HPT
> > at reset time and later (during CAS) create it if it is necessary.
> > 
> > Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
> So, IIRC, we discussed previously that the logical way to do things
> was to, by default, delay HPT allocation until CAS time, and just do
> it at reset time for the case that needs it: hash host with KVM.
> 
> Did you hit a problem with that approach, or is there still work to
> be
> done here?

So what we're doing is assuming radix. Allocate hpt if hash host,
otherwise delay til CAS time and allocate only if guest chose hash.

> 
> > 
> > ---
> > v2:
> > 
> > * This patch has been mostly rewritten to move the late HPT
> > allocation to CAS.
> > This allows a guest to start in radix mode (when it's in real mode)
> > and then
> > change to hash, even if it is a legacy guest and will not call
> > h_register_process_table().
> > * Added an exported function to spapr.c to perform HPT allocation
> > and adjust
> > the vrma if necessary. This makes it possible to allocate the HPT
> > from
> > h_client_architecture_support() in spapr_hcall.c.
> > 
> >  hw/ppc/spapr.c         | 24 +++++++++++++++---------
> >  hw/ppc/spapr_hcall.c   | 10 ++++++++++
> >  include/hw/ppc/spapr.h |  1 +
> >  3 files changed, 26 insertions(+), 9 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index ca3812555f..dfee0f685f 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -1123,6 +1123,17 @@ static void
> > spapr_reallocate_hpt(sPAPRMachineState *spapr, int shift,
> >      }
> >  }
> >  
> > +void spapr_setup_hpt_and_vrma(sPAPRMachineState *spapr)
> > +{
> > +    spapr_reallocate_hpt(spapr,
> > +                     spapr_hpt_shift_for_ramsize(MACHINE(qdev_get_
> > machine())->maxram_size),
> > +                     &error_fatal);
> > +    if (spapr->vrma_adjust) {
> > +        spapr->rma_size = kvmppc_rma_size(spapr_node0_size(),
> > +                                          spapr->htab_shift);
> > +    }
> > +}
> > +
> >  static void find_unknown_sysbus_device(SysBusDevice *sbdev, void
> > *opaque)
> >  {
> >      bool matched = false;
> > @@ -1151,15 +1162,10 @@ static void ppc_spapr_reset(void)
> >      /* Check for unknown sysbus devices */
> >      foreach_dynamic_sysbus_device(find_unknown_sysbus_device,
> > NULL);
> >  
> > -    /* Allocate and/or reset the hash page table */
> > -    spapr_reallocate_hpt(spapr,
> > -                         spapr_hpt_shift_for_ramsize(machine-
> > >maxram_size),
> > -                         &error_fatal);
> > -
> > -    /* Update the RMA size if necessary */
> > -    if (spapr->vrma_adjust) {
> > -        spapr->rma_size = kvmppc_rma_size(spapr_node0_size(),
> > -                                          spapr->htab_shift);
> > +    /* If using KVM with radix mode available, VCPUs can be
> > started
> > +     * without a HPT because KVM will start them in radix mode. */
> > +    if (!(kvm_enabled() && kvmppc_has_cap_mmu_radix())) {
> > +        spapr_setup_hpt_and_vrma(spapr);
> >      }
> >  
> >      qemu_devices_reset();
> > diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> > index 42d20e0b92..cea34073aa 100644
> > --- a/hw/ppc/spapr_hcall.c
> > +++ b/hw/ppc/spapr_hcall.c
> > @@ -1002,6 +1002,16 @@ static target_ulong
> > h_client_architecture_support(PowerPCCPU *cpu,
> >      ov5_updates = spapr_ovec_new();
> >      spapr->cas_reboot = spapr_ovec_diff(ov5_updates,
> >                                          ov5_cas_old, spapr-
> > >ov5_cas);
> > +    if (kvm_enabled()) {
> > +        if (kvmppc_has_cap_mmu_radix()) {
> > +            /* If the HPT hasn't yet been set up (see
> > +             * ppc_spapr_reset()), and it's needed, do it now: */
> I think it's a bit fragile to have here it explicitly mirror the
> logic
> which determines whether the HPT is allocated early.  I'd prefer to
> explicitly test here whether we have allocated an HPT - adding a
> flag,
> if we have to.

We can use the MSB of patb_entry as that flag.

patb_entry & GUEST_RADIX == GUEST_RADIX -> radix, so assume a hpt
hasn't been allocated.

When we do allocate a hpt we know we're not radix, so set
patb_entry &= ~GUEST_RADIX;

Where GUEST_RADIX is the msb in patb_entry which indicates that a guest
is radix.

Essentially patb_entry & GUEST_RADIX cleared mean hash with hpt
allocated, patb_entry & GUEST_RADIX set means radix so assume an hpt
hasn't been allocated. On the hpt allocation path we clear GUEST_RADIX
in patb_entry and when we set GUEST_RADIX we free the hpt.

> 
> > 
> > +            if (!spapr_ovec_test(ov5_updates, OV5_MMU_RADIX)) {
> > +                /* legacy hash or new hash: */
> > +                spapr_setup_hpt_and_vrma(spapr);
> > +            }
> > +        }
> > +    }
> >  
> >      if (!spapr->cas_reboot) {
> >          spapr->cas_reboot =
> > diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> > index f9b17d860a..a30cbc485c 100644
> > --- a/include/hw/ppc/spapr.h
> > +++ b/include/hw/ppc/spapr.h
> > @@ -590,6 +590,7 @@ void spapr_dt_events(sPAPRMachineState *sm,
> > void *fdt);
> >  int spapr_h_cas_compose_response(sPAPRMachineState *sm,
> >                                   target_ulong addr, target_ulong
> > size,
> >                                   sPAPROptionVector *ov5_updates);
> > +void spapr_setup_hpt_and_vrma(sPAPRMachineState *spapr);
> >  sPAPRTCETable *spapr_tce_new_table(DeviceState *owner, uint32_t
> > liobn);
> >  void spapr_tce_table_enable(sPAPRTCETable *tcet,
> >                              uint32_t page_shift, uint64_t
> > bus_offset,