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

["Xie, Huawei" <huawei.xie@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ouyang, Changchun
> Sent: Monday, October 27, 2014 6:56 AM
> To: Thomas Monjalon
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] vhost: Check descriptor number for vector Rx
> 
> Hi Thomas,
> 
> > -----Original Message-----
> > From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > Sent: Monday, October 27, 2014 4:46 PM
> > To: Ouyang, Changchun
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH] vhost: Check descriptor number for vector
> > Rx
> >
> > 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.
> >
> Agree with you, there is something linked between app and driver, but that's due
> to a few things:
> 1.Qume has hard code to confine the total vring size;
> 2.PMD driver need fully fill the dma address of descriptor at setup stage;

Can PMD driver fill its desc ring until all has been filled or  there is no mbuf in pool?

> 3.PMD driver use vector PMD as default path which require more than 32 in the
> burst RX;
> 4.Zero copy need use external buffer directly and set to HW descriptor dma
> address;
> 
> Except for item 3, everything could not be removed or ignored easily.
> As for item 3, I don't know why we set vector PMD as default path, while
> perusing performance, but at the cost of flexibility,
> Here is an example, vhost zero copy need check it and descriptor number. I am
> not sure if the default vector pmd path will affect other app or not.
> 
> On the other hand, without this check, and hint to user,
> User will suffer from this, vhost can't receive packets any more after forwarding
> 31 packets or so,
> and user don't know what really happen behind, also can't easily get the
> information of
> their descriptor is not enough for vector pmd RX.
> Then this is followed by painfully debugging and figuring out the issue.
> 
> While with this check and hint to user, things go smoothly.
> 
> Meanwhile we can waiting for other people's viewpoints.
> 
> Thanks
> Changchun