Re: [dpdk-dev] [PATCH] pci/uio: enable prefetchable resources mapping

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Oct 05, 2017 at 01:06:41AM +0100, Ferruh Yigit wrote:
> On 6/3/2017 11:57 PM, Changpeng Liu wrote:
> > For PCI prefetchable resources, Linux will create a
> > write combined file as well, the library will try
> > to map resourceX_wc file first, if the file does
> > not exist, then it will map resourceX as usual.
> 
> Hi Changpeng,
> 
> Code part looks OK, but can you please describe more why we should try
> write combined resource file first, what is the benefit of using it _wc
> file?
> 
> Thanks,
> ferruh
> 
Also, if we use a write combining resource file, I believe we will use
correct ordering of instructions within our drivers. Does applying this
patch not also mean that we would need memory barriers inside all the
drivers, so as to ensure that we don't have a queue doorbell write
reordered with the descriptor writes? I don't think it's safe to apply
this change on it's own, without driver changes, since all PMDs assume
strong ordering on IA.

Regards,
/Bruce

> 
> > 
> > Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
> > ---
> >  lib/librte_eal/linuxapp/eal/eal_pci_uio.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> > diff --git a/lib/librte_eal/linuxapp/eal/eal_pci_uio.c b/lib/librte_eal/linuxapp/eal/eal_pci_uio.c
> > index fa10329..d9fc20a 100644
> > --- a/lib/librte_eal/linuxapp/eal/eal_pci_uio.c
> > +++ b/lib/librte_eal/linuxapp/eal/eal_pci_uio.c
> > @@ -321,7 +321,7 @@
> >  
> >  	/* update devname for mmap  */
> >  	snprintf(devname, sizeof(devname),
> > -			"%s/" PCI_PRI_FMT "/resource%d",
> > +			"%s/" PCI_PRI_FMT "/resource%d_wc",
> >  			pci_get_sysfs_path(),
> >  			loc->domain, loc->bus, loc->devid,
> >  			loc->function, res_idx);
> > @@ -335,13 +335,22 @@
> >  	}
> >  
> >  	/*
> > -	 * open resource file, to mmap it
> > +	 * open prefetchable resource file first, try to mmap it
> >  	 */
> >  	fd = open(devname, O_RDWR);
> >  	if (fd < 0) {
> > -		RTE_LOG(ERR, EAL, "Cannot open %s: %s\n",
> > -				devname, strerror(errno));
> > -		goto error;
> > +		snprintf(devname, sizeof(devname),
> > +				"%s/" PCI_PRI_FMT "/resource%d",
> > +				pci_get_sysfs_path(),
> > +				loc->domain, loc->bus, loc->devid,
> > +				loc->function, res_idx);
> > +		/* then try to map resource file */
> > +		fd = open(devname, O_RDWR);
> > +		if (fd < 0) {
> > +			RTE_LOG(ERR, EAL, "Cannot open %s: %s\n",
> > +					devname, strerror(errno));
> > +			goto error;
> > +		}
> >  	}
> >  
> >  	/* try mapping somewhere close to the end of hugepages */
> > 
>