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Re: [Xen-devel] [Query] Flow of PCI device dedicated to a domU

On Tue, Jul 22, 2014 at 3:20 PM, Xenbegn developer <xen.begn.dev@xxxxxxxxx> wrote:

On Wed, Jul 16, 2014 at 3:20 PM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
On 16/07/14 07:12, Xenbegn developer wrote:
I am trying to understand the flow of how a PCI device driver in domU works after a PCI device is is assigned to a domU.

a) If a PCI device is assigned to a domU, this device has to be on a PCI bus. So as per my view xen would have to somehow provide a PCI Controller on which this device is attached.
=> Is my assumption correct ? If yes how it is done, No then also How enumeration of this device happens in domU kernel


PV guests have no PCI root ports/bridges; they use devices as single entities knowing that Xen/dom0 takes care of the other bits. HVM guests have their PCI devices attached to the virtual southbridge which is all emulated by Qemu.

b) Is the Configuration space of the PCI device directly accessible to the domU (assuming the kernel accesses it using memory map) ? If not then it is trapped by xen

All configuration space is trap+emulate in Xen, although almost all operations permitted.

c) Who assigns the MSI (addr + value) in the PCI device. If Xen then how Xen does a translation from Physical MSI to guest MSI (where in code)

Xen controls all interrupts on the system, which is why it needs to trap all config accesses to notice when a domain is attempting to change the interrupt information. In that case, Xen fixes up its delivery of interrupts to the guest, but leaves the underlying interrupt information intact.


Linux kernel pci code has a function called __write_msi_msg. This method writes to config space for MSI/MSIX
pci_write_config_dword(dev, pos + PCI_MSI_ADDRESS_LO,
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂÂÂÂ msg->address_lo);

Now as per (b) and (c) it has to be trapped in xen. But in x86/traps.c (guest_io_write) I dont see any MSI handling.
Can you please explain (point to the code) where domain is attempting to change the interrupt information.

Stefano, Can you please comment on it.

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