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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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