Re: [Qemu-devel] [PATCH rebased for-1.8] i386: pc: align gpa<->hpa on 1GB boundary (v6)

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 11 Dec 2013 15:45:29 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> Il 11/12/2013 15:20, Michael S. Tsirkin ha scritto:
> > > It means that its necessary to expose that 3-4GB physical memory region
> > > in QEMU belongs to the same node (that is, guest must be aware that
> > > 3-3.75GB and the tail of RAM are on the same node).
> > > 
> > > So the problem Paolo mentions is fixable.
> 
> I'm not sure if it is fixable.  You need a 2M mountpoint to bind the 3G-4G
> range correctly, a 1G mountpoint for everything else, and QEMU only allows
> to specify one path.
we could do it with hugepage memdev backend.
something like:

-object hugepage-ram,id=mem1gb,size=3G,host-node=0,mem-path=/1gb-hugepage-fs
-device dimm,id=hp1g,memdev=mem1gb,node=0
-object hugepage-ram,id=mem2mb,size=500Mb,host-node=1,mem-path=/2mb-hugepage-fs
-device dimm,id=hp2mb,memdev=mem2mb,node=1

that basically would allow to distribute initial memory in any way user would
like.

> 
> Without Marcelo's patch there is a workaround; if you know the size of the 4G
> hole and configure the first two nodes with unequal sizes.  For example
> 
>    -m 8192 \
>    -object memory-ram,id=ram-node0,size=3840M,hostnode=0 -numa node,memdev=ram-node0 \
>    -object memory-ram,id=ram-node1,size=4352M,hostnode=1 -numa node,memdev=ram-node1
> 
>    RAM address        Host virtual address low bits      Guest physical addresses
>    0M-3840M           0                                  0M-3840M
>    3840M-8192M        0                                  4096M-8448M
> 
> Then you'll waste 1GB of RAM (you'll use 9 hugepages instead of 8), but
> everything will be aligned.  Or you just make your guest 7680M and not waste
> the memory.
> 
> But with Marcelo's patch, ram-node1 will be split in two.  QEMU will try
> to realign the second part of ram-node1, but the result is that the second
> part is misaligned and only the first 256M (the tail of guest physical
> memory) stays aligned:
> 
>    RAM address        Host virtual address low bits      Guest physical addresses
>    0M-3840M           0                                  0M-3840M
>    4096M-8192M        256M                               4096M-8192M
>    3840M-4096M        0                                  8192M-8448M
> 
> So you still waste memory, _and_ get incorrect alignment.
> 
> > Okay so
> > Marcelo - do you ack this patch for 2.0?
> > Paolo - do you re-ack this patch for 2.0?
> 
> I very much prefer Gerd's approach.  2GB low memory for q35 is a bit wasteful,
> but we have some time to fix that before release.
> 
> Paolo
> 


-- 
Regards,
  Igor