Re: [dpdk-dev] [PATCH] vhost: Check descriptor number for vector Rx

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2014-10-25 00:48, Ouyang, Changchun:
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > 2014-10-24 16:38, Ouyang Changchun:
> > > For zero copy, it need check whether RX descriptor num meets the least
> > > requirement when using vector PMD Rx function, and give user more
> > > hints if it fails to meet the least requirement.
> > [...]
> > > --- a/examples/vhost/main.c
> > > +++ b/examples/vhost/main.c
> > > @@ -131,6 +131,10 @@
> > >  #define RTE_TEST_RX_DESC_DEFAULT_ZCP 32   /* legacy: 32, DPDK virt FE: 128. */
> > >  #define RTE_TEST_TX_DESC_DEFAULT_ZCP 64   /* legacy: 64, DPDK virt FE: 64.  */
> > >
> > > +#ifdef RTE_IXGBE_INC_VECTOR
> > > +#define VPMD_RX_BURST         32
> > > +#endif
> > > +
> > >  /* Get first 4 bytes in mbuf headroom. */  #define
> > > MBUF_HEADROOM_UINT32(mbuf) (*(uint32_t *)((uint8_t *)(mbuf) \
> > >  		+ sizeof(struct rte_mbuf)))
> > > @@ -792,6 +796,19 @@ us_vhost_parse_args(int argc, char **argv)
> > >  		return -1;
> > >  	}
> > >
> > > +#ifdef RTE_IXGBE_INC_VECTOR
> > > +	if ((zero_copy == 1) && (num_rx_descriptor <= VPMD_RX_BURST)) {
> > > +		RTE_LOG(INFO, VHOST_PORT,
> > > +			"The RX desc num: %d is too small for PMD to work\n"
> > > +			"properly, please enlarge it to bigger than %d if\n"
> > > +			"possible by the option: '--rx-desc-num <number>'\n"
> > > +			"One alternative is disabling RTE_IXGBE_INC_VECTOR\n"
> > > +			"in config file and rebuild the libraries.\n",
> > > +			num_rx_descriptor, VPMD_RX_BURST);
> > > +		return -1;
> > > +	}
> > > +#endif
> > > +
> > >  	return 0;
> > >  }
> > 
> > I feel there is a design problem here.
> > An application shouldn't have to care about the underlying driver.
> 
> For most of other applications, as their descriptor numbers are set as big enough(1024 or so) ,
> So there is no need to check the descriptor number at the early stage of running.
> 
> But for vhost zero copy(note vhost one copy also has 1024 descriptor number) has the default 
> descriptor number of 32.
> Why use 32? 
> because vhost zero copy implementation (working as backend) need support dpdk based app which use pmd virtio-net driver,
> And also need support linux legacy virtio-net based application.  
> When it is the linux legacy virtio-net case, on one side the qemu has hard code to confine the total virtio descriptor size to 256, 
> On other side, legacy virtio use half of them as virtio header, and then only another half i.e. 128 descriptors are available to use as real buffer.
> 
> In PMD mode, all HW descriptors need to be filled DMA address in the rx initial stage, otherwise there is probably exceptional in rx process.
> Based on that, we need use really limited virtio buffer to fully fill all hw descriptor DMA address,
> Or in other word, the available virtio descriptor size will determine the total mbuf size and hw descriptor size in the case of zero copy,
> 
> Tune and find that 32 is the suitable value for vhost zero copy to work properly when it legacy linux virtio case.
> Another factor to reduce the value to 32, is that mempool use ring to accommodate the mbuf, it cost one to flag the ring head/tail,
> And there are some other overheads like temporary mbufs(size as RX_BURST) when rx.
> Note that number descriptor should need power 2.   
> 
> Why the change occur at this moment?
> Recently the default rx function is modified into vector RX function, while it use non-vector mode (scalar mode) Rx previously,
> Vector RX function need more than 32 descriptor to work properly,  but scalar mode RX hasn't this limitation.
> 
> As the RX function is changeable(you can use vector mode or non-vector), and descriptor number can also be changed.
> So here in the vhost app, check if they match to make sure all things could work normally, and give some hints if they don't match.
> 
> Hope the above could make it a bit clearer. :-)

Thank you for your explanation.
Your fix shows that driver and application are tightly linked.
It's a design issue. As I said:
"An application shouldn't have to care about the underlying driver."
I didn't dig enough in vhost to suggest a good fix but I'm sure
someone could have an idea.

-- 
Thomas