Re: [Qemu-devel] [PATCH 2/2] pc: memhp: force gaps between DIMM's GPA

[Igor Mammedov <imammedo@redhat.com>]

On Sun, 27 Sep 2015 13:48:21 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Fri, Sep 25, 2015 at 03:53:12PM +0200, Igor Mammedov wrote:
> > mapping DIMMs non contiguously allows to workaround
> > virtio bug reported earlier:
> > http://lists.nongnu.org/archive/html/qemu-devel/2015-08/msg00522.html
> > in this case guest kernel doesn't allocate buffers
> > that can cross DIMM boundary keeping each buffer
> > local to a DIMM.
> > 
> > Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> > benefit of this workaround is that no guest side
> > changes are required.
> 
> That's a hard requirement, I agree.
> 
> 
> > ---
> >  hw/i386/pc.c         | 4 +++-
> >  hw/i386/pc_piix.c    | 3 +++
> >  hw/i386/pc_q35.c     | 3 +++
> >  include/hw/i386/pc.h | 2 ++
> >  4 files changed, 11 insertions(+), 1 deletion(-)
> 
> Aren't other architectures besides PC ever affected?
> Do they all allocate all of memory contigious in HVA space?
I'm not sure about other targets I've CCed interested parties.

> 
> Also - does the issue only affect hotplugged memory?
Potentially it affects -numa memdev=foo, but however I've
tried I wasn't able to reproduce. We could do it as
separate workaround later if it would affect someone
and virtio is not fixed to handle split buffers by that time.

 
> Can't the patch be local to pc-dimm (except maybe the
> backwards compatibility thing)?
I think decision about using gaps and its size
should be done by board and not generic pc-dimm.


> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index 91d134c..c462c4e 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -1629,6 +1629,7 @@ static void pc_dimm_plug(HotplugHandler *hotplug_dev,
> >      HotplugHandlerClass *hhc;
> >      Error *local_err = NULL;
> >      PCMachineState *pcms = PC_MACHINE(hotplug_dev);
> > +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
> >      PCDIMMDevice *dimm = PC_DIMM(dev);
> >      PCDIMMDeviceClass *ddc = PC_DIMM_GET_CLASS(dimm);
> >      MemoryRegion *mr = ddc->get_memory_region(dimm);
> > @@ -1644,7 +1645,8 @@ static void pc_dimm_plug(HotplugHandler *hotplug_dev,
> >          goto out;
> >      }
> >  
> > -    pc_dimm_memory_plug(dev, &pcms->hotplug_memory, mr, align, 0, &local_err);
> > +    pc_dimm_memory_plug(dev, &pcms->hotplug_memory, mr, align,
> > +                        pcmc->inter_dimm_gap, &local_err);
> >      if (local_err) {
> >          goto out;
> >      }
> > diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> > index 3ffb05f..3165667 100644
> > --- a/hw/i386/pc_piix.c
> > +++ b/hw/i386/pc_piix.c
> > @@ -457,11 +457,13 @@ static void pc_xen_hvm_init(MachineState *machine)
> >  
> >  static void pc_i440fx_machine_options(MachineClass *m)
> >  {
> > +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> >      m->family = "pc_piix";
> >      m->desc = "Standard PC (i440FX + PIIX, 1996)";
> >      m->hot_add_cpu = pc_hot_add_cpu;
> >      m->default_machine_opts = "firmware=bios-256k.bin";
> >      m->default_display = "std";
> > +    pcmc->inter_dimm_gap = PC_2MB_DIMM_GAP;
> >  }
> >  
> >  static void pc_i440fx_2_5_machine_options(MachineClass *m)
> > @@ -482,6 +484,7 @@ static void pc_i440fx_2_4_machine_options(MachineClass *m)
> >      m->alias = NULL;
> >      m->is_default = 0;
> >      pcmc->broken_reserved_end = true;
> > +    pcmc->inter_dimm_gap = 0;
> >      SET_MACHINE_COMPAT(m, PC_COMPAT_2_4);
> >  }
> >  
> > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > index 1b7d3b6..8ad6687 100644
> > --- a/hw/i386/pc_q35.c
> > +++ b/hw/i386/pc_q35.c
> > @@ -360,6 +360,7 @@ static void pc_compat_1_4(MachineState *machine)
> >  
> >  static void pc_q35_machine_options(MachineClass *m)
> >  {
> > +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> >      m->family = "pc_q35";
> >      m->desc = "Standard PC (Q35 + ICH9, 2009)";
> >      m->hot_add_cpu = pc_hot_add_cpu;
> > @@ -368,6 +369,7 @@ static void pc_q35_machine_options(MachineClass *m)
> >      m->default_display = "std";
> >      m->no_floppy = 1;
> >      m->no_tco = 0;
> > +    pcmc->inter_dimm_gap = PC_2MB_DIMM_GAP;
> >  }
> >  
> >  static void pc_q35_2_5_machine_options(MachineClass *m)
> > @@ -385,6 +387,7 @@ static void pc_q35_2_4_machine_options(MachineClass *m)
> >      pc_q35_2_5_machine_options(m);
> >      m->alias = NULL;
> >      pcmc->broken_reserved_end = true;
> > +    pcmc->inter_dimm_gap = 0;
> >      SET_MACHINE_COMPAT(m, PC_COMPAT_2_4);
> >  }
> >  
> > diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> > index 6896328..dd6b34a 100644
> > --- a/include/hw/i386/pc.h
> > +++ b/include/hw/i386/pc.h
> > @@ -50,6 +50,7 @@ struct PCMachineState {
> >  #define PC_MACHINE_SMM              "smm"
> >  #define PC_MACHINE_ENFORCE_ALIGNED_DIMM "enforce-aligned-dimm"
> >  
> > +#define PC_2MB_DIMM_GAP (1ULL << 21)
> >  /**
> >   * PCMachineClass:
> >   * @get_hotplug_handler: pointer to parent class callback @get_hotplug_handler
> 
> Seems somewhat arbitrary. It's aligned later - so won't a 1 byte gap be enough?
1 byte should be also enough, since effectively it would kick alignment adjustment.

The reason why I've picked 2Mb is that QEMU ram allocator allocates
2Mb granularity.

> 
> > @@ -60,6 +61,7 @@ struct PCMachineClass {
> >  
> >      /*< public >*/
> >      bool broken_reserved_end;
> > +    uint64_t inter_dimm_gap;
> >      HotplugHandler *(*get_hotplug_handler)(MachineState *machine,
> >                                             DeviceState *dev);
> >  };
> > -- 
> > 1.8.3.1