Re: [Qemu-devel] The results of lspci are inconsistent between vfio reset pci devices and reset devices by sysfs interafce

["Wuzongyong (Euler Dept)" <cordius.wu@huawei.com>]

> > 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