Re: [Qemu-devel] vfio failure with intel 760p 128GB nvme

[Alex Williamson <alex.williamson@redhat.com>]

On Sat, 1 Dec 2018 10:52:21 -0800 (PST)
Dongli Zhang <dongli.zhang@oracle.com> wrote:

> Hi,
> 
> I obtained below error when assigning an intel 760p 128GB nvme to guest via
> vfio on my desktop:
> 
> qemu-system-x86_64: -device vfio-pci,host=0000:01:00.0: vfio 0000:01:00.0: failed to add PCI capability 0x11[0x50]@0xb0: table & pba overlap, or they don't fit in BARs, or don't align
> 
> 
> This is because the msix table is overlapping with pba. According to below
> 'lspci -vv' from host, the distance between msix table offset and pba offset is
> only 0x100, although there are 22 entries supported (22 entries need 0x160).
> Looks qemu supports at most 0x800.
> 
> # sudo lspci -vv
> ... ...
> 01:00.0 Non-Volatile memory controller: Intel Corporation Device f1a6 (rev 03) (prog-if 02 [NVM Express])
> 	Subsystem: Intel Corporation Device 390b
> ... ...
> 	Capabilities: [b0] MSI-X: Enable- Count=22 Masked-
> 		Vector table: BAR=0 offset=00002000
> 		PBA: BAR=0 offset=00002100
> 
> 
> 
> A patch below could workaround the issue and passthrough nvme successfully.
> 
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 5c7bd96..54fc25e 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -1510,6 +1510,11 @@ static void vfio_msix_early_setup(VFIOPCIDevice *vdev, Error **errp)
>      msix->pba_offset = pba & ~PCI_MSIX_FLAGS_BIRMASK;
>      msix->entries = (ctrl & PCI_MSIX_FLAGS_QSIZE) + 1;
>  
> +    if (msix->table_bar == msix->pba_bar &&
> +        msix->table_offset + msix->entries * PCI_MSIX_ENTRY_SIZE > msix->pba_offset) {
> +        msix->entries = (msix->pba_offset - msix->table_offset) / PCI_MSIX_ENTRY_SIZE;
> +    }
> +
>      /*
>       * Test the size of the pba_offset variable and catch if it extends outside
>       * of the specified BAR. If it is the case, we need to apply a hardware
> 
> 
> Would you please help confirm if this can be regarded as bug in qemu, or issue
> with nvme hardware? Should we fix thin in qemu, or we should never use such buggy
> hardware with vfio?

It's a hardware bug, is there perhaps a firmware update for the device
that resolves it?  It's curious that a vector table size of 0x100 gives
us 16 entries and 22 in hex is 0x16 (table size would be reported as
0x15 for the N-1 algorithm).  I wonder if there's a hex vs decimal
mismatch going on.  We don't really know if the workaround above is
correct, are there really 16 entries or maybe does the PBA actually
start at a different offset?  We wouldn't want to generically assume
one or the other.  I think we need Intel to tell us in which way their
hardware is broken and whether it can or is already fixed in a firmware
update.  Thanks,

Alex