[Xen-devel] Accessing PCI BAR space directly from XEN hypervisor

[veerasena reddy <veeruyours@xxxxxxxxx>]

Hi,

I am doing an experiment to see if I can get more throughput by moving the Tx ring update and doorbell ring mechanism to hypervisor and provide a hypercall to HVM Guest. When there is a Tx packet ready, the HVM guest invokes the hypercall along with the packet address as argument. The hypercall function will write the packet address to the Tx ring and rings the doorbell directly without involving dom0.

AS part of this, i need to access IO Memory BAR region of the PCI network device from XEN hypervisor (hypercall handler). For this, i tried converting the BAR Physical address (captured from lspci as shown below) of the PCI device to MFN, but got all F's (invalid entry). Even pfn_valid(0xf0904000) failed on this address. Where as I could successfully map a dom0 page (allocated using vmalloc()) and access in hypervisor using map_domain_page(mfn).

Could any one please suggest how to access the BAR regions of a PCI device from hypervisor? how to do mapping and read/write operations?

Also, Could you please explain the difference between mapping locally allocated pages and PCI BAR memory of dom0 in hypervisor?

Thanks in advance.

Regards,
VSR.
==========================
[root@fc13 xen]# lspci -vvs 09:00.0
09:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 03)
    Subsystem: Lenovo Device 2131
    Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx+
    Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
    Latency: 0, Cache Line Size: 64 bytes
    Interrupt: pin A routed to IRQ 1247
    Region 0: I/O ports at 4000 [size=256]
    Region 2: Memory at f0904000 (64-bit, prefetchable) [size=4K]
    Region 4: Memory at f0900000 (64-bit, prefetchable) [size=16K]
    [virtual] Expansion ROM at f0920000 [disabled] [size=128K]
===========================
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