Re: [PATCH 2/3] KVM: Emulate MSI-X table in kernel

[Avi Kivity <avi@redhat.com>]

On 01/17/2011 02:18 PM, Sheng Yang wrote:
> >  >  +
> >  >  +	if (copy_to_user((void __user *)(entry_base + offset), val, len))
> >  >  +		goto out;
> >
> >  Instead of copying to/from userspace (which is subject to swapin,
> >  unexpected values), you could include the guest written value in a
> >  kvm_run structure, along with address. Qemu-kvm would use that to
> >  synchronize its copy of the table, on KVM_EXIT_MSIX_ROUTING_UPDATE exit.
>
> We want to acelerate MSI-X mask bit accessing, which won't exit to userspace in
> the most condition. That's the cost we want to optimize. Also it's possible to
> userspace to read the correct value of MMIO(but mostly userspace can't write to it
> in order to prevent synchronize issue).

It's also good to have the values in just one place; using userspace 
makes it easy for both the kernel and userspace to see the values (and 
set them after migration, if/when we extend this to virtio).

-- 
error compiling committee.c: too many arguments to function