Re: [Qemu-devel] [PATCH] pc: memhp: enforce minimal 128Mb alignment for pc-dimm

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 27, 2015 at 02:24:17PM -0200, Eduardo Habkost wrote:
> On Tue, Oct 27, 2015 at 11:08:26AM +0200, Michael S. Tsirkin wrote:
> > On Mon, Oct 26, 2015 at 04:33:18PM -0200, Eduardo Habkost wrote:
> > > On Mon, Oct 26, 2015 at 09:42:05AM +0100, Igor Mammedov wrote:
> > > > commit aa8580cd "pc: memhp: force gaps between DIMM's GPA"
> > > > regressed memory hot-unplug for linux guests triggering
> > > > following BUGON
> > > >  =====
> > > >  kernel BUG at mm/memory_hotplug.c:703!
> > > >  ...
> > > >  [<ffffffff81385fa7>] acpi_memory_device_remove+0x79/0xa5
> > > >  [<ffffffff81357818>] acpi_bus_trim+0x5a/0x8d
> > > >  [<ffffffff81359026>] acpi_device_hotplug+0x1b7/0x418
> > > >  ===
> > > >     BUG_ON(phys_start_pfn & ~PAGE_SECTION_MASK);
> > > >  ===
> > > > 
> > > > reson for it is that x86-64 linux guest supports memory
> > > > hotplug in chunks of 128Mb and memory section also should
> > > > be 128Mb aligned.
> > > > However gaps forced between 128Mb DIMMs with backend's
> > > > natural alignment of 2Mb make the 2nd and following
> > > > DIMMs not being aligned on 128Mb boundary as it was
> > > > originally. To fix regression enforce minimal 128Mb
> > > > alignment like it was done for PPC.
> > > > 
> > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > > ---
> > > >  hw/i386/pc.c | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > > > index 3d958ba..cd68169 100644
> > > > --- a/hw/i386/pc.c
> > > > +++ b/hw/i386/pc.c
> > > > @@ -1610,6 +1610,8 @@ void ioapic_init_gsi(GSIState *gsi_state, const char *parent_name)
> > > >      }
> > > >  }
> > > >  
> > > > +#define MIN_DIMM_ALIGNMENT (1ULL << 27) /* 128Mb */
> > > 
> > > If you send a new version, could you include the explanation for the
> > > 128MB value as a comment above the macro definition?
> > 
> > The issue is that there's no good explanation yet.  It's just something
> > that seems to work for current linux.  Why does linux do it, and what
> > basis does it have in hardware, IIUC we don't know.
> 
> We just need an explanation to why we chose that value, even if we don't
> know yet why it works. Even "this is the only value we ever tested and
> it seems to work, good luck figuring out why" would be better than no
> explanation, IMO.

Not by much though :)

> -- 
> Eduardo