Re: [dpdk-dev] [PATCH v2] eal/bus: use RTE_IOVA_PA only if phys addresses are available

[Thomas Monjalon <thomas@monjalon.net>]

17/09/2018 12:33, Burakov, Anatoly:
> On 07-Sep-18 4:58 PM, Darek Stojaczyk wrote:
> > When neither RTE_IOVA_VA nor RTE_IOVA_PA was explicitly
> > requested, DPDK would currently fallback to the default
> > RTE_IOVA_PA mode and possibly encounter a failure later
> > on if running as a non-priviledged user. Attempting to
> > use RTE_IOVA_VA if no phys addresses are available may
> > help in this case.
> > 
> > Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
> > ---
> > Changes since v1:
> >   * added a missing rte_memory.h include
> > 
> >   lib/librte_eal/common/eal_common_bus.c | 19 +++++++++++++++----
> >   1 file changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/lib/librte_eal/common/eal_common_bus.c b/lib/librte_eal/common/eal_common_bus.c
> > index 0943851cc..68c581b8a 100644
> > --- a/lib/librte_eal/common/eal_common_bus.c
> > +++ b/lib/librte_eal/common/eal_common_bus.c
> > @@ -37,6 +37,7 @@
> >   #include <rte_bus.h>
> >   #include <rte_debug.h>
> >   #include <rte_string_fns.h>
> > +#include <rte_memory.h>
> >   
> >   #include "eal_private.h"
> >   
> > @@ -236,9 +237,19 @@ rte_bus_get_iommu_class(void)
> >   			mode |= bus->get_iommu_class();
> >   	}
> >   
> > -	if (mode != RTE_IOVA_VA) {
> > -		/* Use default IOVA mode */
> > -		mode = RTE_IOVA_PA;
> > +	if (mode == RTE_IOVA_VA)
> > +		return RTE_IOVA_VA;
> > +
> > +	if (mode & RTE_IOVA_PA) {
> > +		/* Not all buses support RTE_IOVA_VA, fallback to RTE_IOVA_PA */
> > +		return RTE_IOVA_PA;
> > +	}
> > +
> > +	if (rte_eal_using_phys_addrs()) {
> > +		/* Default to RTE_IOVA_PA only if it's supported */
> > +		return RTE_IOVA_PA;
> >   	}
> > -	return mode;
> > +
> > +	/* Since RTE_IOVA_PA is unsupported, fallback to RTE_IOVA_VA */
> > +	return RTE_IOVA_VA;
> >   }
> > 
> 
> This is a good change, however I think that this is too pessimistic. If 
> i don't have any devices that explictly require IOVA_PA, i should be 
> running in IOVA_VA mode.
> 
> This of course doesn't take hotplug into account, so a command-line 
> switch to force one or the other should also be available.
> 
> For example, at startup, i might have devices bound to VFIO, so IOVA_VA 
> mode is picked. However, even though at a time of startup none of the 
> devices require physical addresses, i also know that i might later 
> hotplug a device that requires IOVA_PA (leaving the question of hotplug 
> brokenness aside for now...) - currently, this scenario will not work, 
> as i will be forced to use IOVA_VA mode unless i happen to have a 
> IOVA_PA device available at startup.
> 
> Similarly, if i'm running DPDK as root but am only using virtual devices 
> like pcap, i should be able to force DPDK into using VA addresses [*], 
> yet currently i will be forced to use IOVA_PA if i don't *also* have a 
> few devices bound exclusively to VFIO.
> 
> [*] Do we have vdev devices that require IOVA_PA? I can't think of any...

If running as root, what is the benefit of using virtual addresses?