From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH 2/3] KVM: Emulate MSI-X table in kernel
Date: Mon, 17 Jan 2011 10:48:16 -0200
Message-ID: <20110117124816.GB8985@amt.cnet>
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 <20110117115447.GA8145@amt.cnet>
 <201101172018.22478.sheng@linux.intel.com>
 <4D3433A3.2010305@redhat.com>
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Cc: Sheng Yang <sheng@linux.intel.com>,
	"Michael S. Tsirkin" <mst@redhat.com>, kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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On Mon, Jan 17, 2011 at 02:18:43PM +0200, Avi Kivity wrote:
> 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).

Right, thats an advantage, but:

- How can userspace ever synchronize with updates by the kernel 
  to the MSI-X entry?
- Reading/writing to the userspace area must be done carefully,
  values must be validated before used.
- Swapping issue (minor?).