Re: [dpdk-dev] [PATCH] bus/pci: fix mmap PCI resource

["Zhang, AlvinX" <alvinx.zhang@intel.com>]

Hi Chenbo,

Thanks your comments.

> -----Original Message-----
> From: Xia, Chenbo <chenbo.xia@intel.com>
> Sent: Wednesday, July 8, 2020 9:58 PM
> To: Zhang, AlvinX <alvinx.zhang@intel.com>; dev@dpdk.org
> Cc: Xing, Beilei <beilei.xing@intel.com>; Guo, Jia <jia.guo@intel.com>; David
> Marchand <david.marchand@redhat.com>; Tal Shnaiderman
> <talshn@mellanox.com>; Burakov, Anatoly <anatoly.burakov@intel.com>;
> thomas@monjalon.net
> Subject: RE: [dpdk-dev] [PATCH] bus/pci: fix mmap PCI resource
> 
> Hi Alvin,
> 
> CC the maintainers. Comments below.
> 
> > -----Original Message-----
> > From: dev <dev-bounces@dpdk.org> On Behalf Of alvinx.zhang@intel.com
> > Sent: Wednesday, July 8, 2020 5:25 PM
> > To: dev@dpdk.org
> > Cc: Xing, Beilei <beilei.xing@intel.com>; Guo, Jia <jia.guo@intel.com>
> > Subject: [dpdk-dev] [PATCH] bus/pci: fix mmap PCI resource
> >
> > From: Alvin Zhang <alvinx.zhang@intel.com>
> >
> > When mapping a PCI BAR containing an MSI-X table, some devices do not
> > need to actually map this BAR or only need to map part of them, which
> > may cause the mapping to fail. Now some checks are added and a
> > non-NULL initial value is set to the variable to avoid this situation.
> >
> > Fixes: 2fd3567e5425 ("pci: use OS generic memory mapping functions")
> > Cc: talshn@mellanox.com
> >
> > Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
> > ---
> >  drivers/bus/pci/linux/pci_vfio.c | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/bus/pci/linux/pci_vfio.c
> > b/drivers/bus/pci/linux/pci_vfio.c
> > index fdeb9a8..9143bfc 100644
> > --- a/drivers/bus/pci/linux/pci_vfio.c
> > +++ b/drivers/bus/pci/linux/pci_vfio.c
> > @@ -547,6 +547,14 @@
> >  			bar_index,
> >  			memreg[0].offset, memreg[0].size,
> >  			memreg[1].offset, memreg[1].size);
> > +
> > +		if (memreg[0].size == 0 && memreg[1].size == 0) {
> > +			/* No need to map this BAR */
> > +			RTE_LOG(DEBUG, EAL, "Skipping BAR%d\n", bar_index);
> > +			bar->size = 0;
> > +			bar->addr = 0;
> > +			return 0;
> > +		}
> 
> I'm not sure if this corner case will happen. If you confirmed it, Just ignore this.

In theory, it is entirely possible if the misx-table size is equal to the bar size.

> 
> >  	} else {
> >  		memreg[0].offset = bar->offset;
> >  		memreg[0].size = bar->size;
> > @@ -556,7 +564,9 @@
> >  	bar_addr = mmap(bar->addr, bar->size, 0, MAP_PRIVATE |
> >  			MAP_ANONYMOUS | additional_flags, -1, 0);
> >  	if (bar_addr != MAP_FAILED) {
> > -		void *map_addr = NULL;
> > +		/* Set non NULL initial value for in case of no PCI mapping */
> > +		void *map_addr = bar_addr;
> > +
> 
> I see the issue that this patch wants to fix is based on an old kernel.
> In older vfio-pci kernel module, MSI related reg cannot be mmaped in userspace
> while in newer kernel it can be. That's why sometimes it cannot be reproduced
> (https://bugs.dpdk.org/show_bug.cgi?id=503)
> 
> So under the condition of old kernel, there could be an example that Memreg 0
> has size 0 but Memreg 1 has non-zero size, which leads to Memreg 1 cannot be
> mmaped.

Yes, it is.

> 
> So I'm fine with this part of code change. As this issue is blocking test, we should
> do fast confirm and review.
> 
> Thanks,
> Chenbo
> 
> >  		if (memreg[0].size) {
> >  			/* actual map of first part */
> >  			map_addr = pci_map_resource(bar_addr, vfio_dev_fd,
> > --
> > 1.8.3.1