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Re: [Xen-devel] [question] Does HVM domain support xen-pcifront/xen-pciback?

On 02/07/2012 01:58 AM, Konrad Rzeszutek Wilk wrote:
On Mon, Feb 06, 2012 at 04:32:05PM +0800, Kai Huang wrote:

I see in pcifront_init, if domain is not PV domain, pcifront_init just
returns error. So seems HVM domain does not support
xen-pcifront/xen-pciback mechanism? If it is true, why? I think
Yup. B/c the only thing that the PV PCI protocol does is enable
PCI configuration emulation. And if you boot an HVM guest - QEMU does
that already.

I heard qemu does not support PCIE simulation, and Xen does not provides MMIO mechanism but only legacy IO port mechanism to guest for configuration space access. Is this true?

If using IO port mechanism, we can only access first 256B of configuration space, but if using PV PCI protocol, we will not have such limitation. I think this is an advantage of PCI PV protocol. Of course if Xen provides MMIO mechanism to guest for configuration space, it will not have this limitation too.

technically there's nothing that can block to support pcifront/pciback
in HVM, and for performance reason, there will be benefits if HVM
supports PV PCI operation.
Nope. The PCI operations are just for writting configuration deta in the
PCI space. Whcih is done mostly when a driver starts/stops and no more.

The performance is with interrupts and how to inject them in a guest - and
in there the PV is much faster than HVM due to the emulation layer complexity.
However, work by Stefano on making that path shorter has made the interrupt
injection much much faster.
I think PV PCI protocol can be used for other purpose in the future, such as PF/VF communication. In this case it will be better if HVM domain can support PV PCI protocol.


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