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Hi,

I start a virtual machine with commandline: /usr/libexec/qemu-kvm --enable-kvm -smp 8 -m 8192 -device vfio-pci,host=0000:81:00.0

Then I pause the qemu process before executing the main_loop function by gdb. At this moment, lspci shows the regions are disabled like below: 81:00.0 3D controller: NVIDIA Corporation GP100GL [Tesla P100 PCIe 16GB] (rev a1) Subsystem: NVIDIA Corporation Device 118f Physical Slot: 0-6 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- Interrupt: pin A routed to IRQ 35 NUMA node: 1 Region 0: Memory at c8000000 (32-bit, non-prefetchable) [disabled] [size=16M] Region 1: Memory at 27800000000 (64-bit, prefetchable) [disabled] [size=16G] Region 3: Memory at 27c00000000 (64-bit, prefetchable) [disabled] [size=32M]

But after the command: echo 1 > /sys/bus/pci/devices/0000:81:00.0/reset lspci shows the regions are *not* disabled: 81:00.0 3D controller: NVIDIA Corporation GP100GL [Tesla P100 PCIe 16GB] (rev a1) Subsystem: Huawei Technologies Co., Ltd. Device 2061 Physical Slot: 0-6 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: 32 bytes Interrupt: pin A routed to IRQ 7 NUMA node: 1 Region 0: Memory at c8000000 (32-bit, non-prefetchable) [size=16M] Region 1: Memory at 27800000000 (64-bit, prefetchable) [size=16G] Region 3: Memory at 27c00000000 (64-bit, prefetchable) [size=32M]

AFAIK, qemu performs vfio_pci_reset like the below callstack: Qemu: vfio_pci_reset ioctl(vdev->vbasedev.fd, VFIO_DEVICE_RESET) Kernel: vfio_pci_ioctl pci_try_reset_function __pci_reset_function_locked pci_parent_bus_reset pci_reset_bridge_secondary_bus

and write 1 to the reset interface of sysfs go through the path: Kernel: reset_store pci_reset_function __pci_reset_function_locked pci_parent_bus_reset pci_reset_bridge_secondary_bus

So seem that these two methods are same actually, I am confused why the results are inconsistent.

Maybe there's a misunderstanding here, the kernel PCI reset functions save and restore config space around the reset. The intention of the reset is to re-init the internal state of the device while preserving (via save+restore) the config space. The BARs being disabled is simply a matter of the Memory bit in the Command register being unset (note Mem-). Whether this is indicative of some issue depends on whether the state before reset matches the state after reset, not that the states after two different paths of triggering a reset are identical.

vfio-pci will hand off the device to the user (QEMU) disabled, so the states in the first example make sense to me. In the second case, it's not clear what the starting state is for the device. Was this reset performed from the starting point of the first case or is the device in some arbitrary, unknown state prior to reset? Thanks,

Alex In the second case, the reset was performed from the starting point of the first case. IOW, the states before the two cases are identical, I think. The only difference I can think of is the qemu process will perform twice reset, one occurs when vfio open the device' fd and the other one occurs as I mentioned above.

Thanks, Wu Zongyong

On Wed, 10 Oct 2018 01:47:10 +0000 "Wuzongyong (Euler Dept)" <cordius.wu@huawei.com> wrote:

You're right. The initial states are not identical. I found the function vfio_pci_pre_reset in qemu. /* * Stop any ongoing DMA by disconecting I/O, MMIO, and bus master. * Also put INTx Disable in known state. */ cmd = vfio_pci_read_config(pdev, PCI_COMMAND, 2); cmd &= ~(PCI_COMMAND_IO | PCI_COMMAND_MEMORY | PCI_COMMAND_MASTER | PCI_COMMAND_INTX_DISABLE); vfio_pci_write_config(pdev, PCI_COMMAND, cmd, 2);

So the behaviors between the two reset are inconsistent.

Then I wonder whether the operation is necessary here? Could I enable the Memory bit in the Command register in vfio_pci_post_reset, because I want to write regions of PCI devices after reset.

One reset is done by the kernel to try to put the device into a known clean state before allowing the user access to it, the other is done by QEMU as part of the initial machine reset. I suppose we could special case the initial machine reset, but it seems perhaps risky and unnecessary. QEMU is the driver here, it can certainly enable MMIO on the device and there are some examples in the QEMU code where MMIO is enabled to interact with the device, see vfio_radeon_reset() for example. The device driver in the guest or the VM firmware should be the one to enable the device for VM usage though, QEMU should provide the device to the VM in a power-on default state, or as close as we can reasonably get to that. Thanks, Alex