Re: [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

[Paul Brook]

> > That sounds like it's likely to come back and bite you. The guest has no
> > idea which areas of ram happen to be contiguous on the host.
> 
> Practically speaking, with target-i386 anything that is contiguous in
> guest physical memory is contiguous in the host address space provided
> it's ram.
> 
> These assumptions are important.  I have a local branch (that I'll send
> out soon) that implements a ram API and converted virtio to make use of
> it.  I'm seeing a ~50% increase in tx throughput.

IMO supporting discontiguous regions is a requirement. target-i386 might get 
away with contiguous memory, because it omits most of the board level details. 
For everything else I'd expect this to be a real problem.

I'm not happy about the not-remapable assumption either.  In my experience 
this is fairly common.  In fact many real x86 machines have this capability 
(to workaround the 4G PCI hole).

By my reading the ppc440_bamboo board fails both your assumptions.
I imagine the KVM-PPC folks would be upset if you decided that virtio no 
longer worked on this board.

This is all somewhat disappointing, given virtio is supposed to be a DMA based 
architecture, and not rely on shared memory semantics.

Paul