Message-ID: <20090929203018.GG3958@sequoia.sous-sol.org>
Date:	Tue, 29 Sep 2009 13:30:18 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Bhavesh Davda <bhavesh@...are.com>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Shreyas Bhatewara <sbhatewara@...are.com>,
	"pv-drivers@...are.com" <pv-drivers@...are.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Greg Kroah-Hartman <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeff Garzik <jgarzik@...ox.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Pv-drivers] [PATCH 2.6.31-rc9] net: VMware virtual Ethernet
	NIC driver: vmxnet3

* Bhavesh Davda (bhavesh@...are.com) wrote:
> Hi Chris,
> 
> Thanks a bunch for your really thorough review! I'll answer some of your questions here. Shreyas can respond to your comments about some of the coding style/comments/etc. in a separate mail.

The style is less important at this stage, but certainly eases review
to make it more consistent w/ Linux code.  The StudlyCaps, extra macros
(screaming caps) and inconistent space/tabs are visual distractions,
that's all.

> > >         INTx, MSI, MSI-X (25 vectors) interrupts
> > >         16 Rx queues, 8 Tx queues
> > 
> > Driver doesn't appear to actually support more than a single MSI-X
> > interrupt.
> > What is your plan for doing real multiqueue?
> 
> When we first wrote the driver a couple of years ago, Linux lacked proper multiqueue support, hence we chose to use only a single queue though the emulated device does support 16 Rx and 8 Tx queues, and 25 MSI-X vectors: 16 for Rx, 8 for Tx and 1 for other asynchronous event notifications, by design. Actually a driver can repurpose any of the 25 vectors for any notifications; just explaining the rationale for desiging the device with 25 MSI-X vectors.

I see, thanks.

> We do have an internal prototype of a Linux vmxnet3 driver with 4 Tx queues and 4 Rx queues, using 9 MSI-X vectors, but it needs some work before calling it production ready.

I'd expect once you switch to alloc_etherdev_mq(), make napi work per
rx queue, and fix MSI-X allocation (all needed for 4/4), you should
have enough to support the max of 16/8 (IOW, 4/4 still sounds like an
aritificial limitation).

> > How about GRO conversion?
> 
> Looks attractive, and we'll work on that in a subsequent patch. Again, when we first wrote the driver, the NETIF_F_GRO stuff didn't exist in Linux.

OK, shouldn't be too much work.

Another thing I forgot to mention is that net_device now has
net_device_stats in it.  So you shouldn't need net_device_stats in
vmxnet3_adapter.

> > Also, heavy use of BUG_ON() (counted 51 of them), are you sure that
> > none
> > of them can be triggered by guest or remote (esp. the ones that happen
> > in interrupt context)?  Some initial thoughts below.
> 
> We'll definitely audit all the BUG_ONs again to make sure they can't be exploited.
> 
> > > --- /dev/null
> > > +++ b/drivers/net/vmxnet3/upt1_defs.h
> > > +#define UPT1_MAX_TX_QUEUES  64
> > > +#define UPT1_MAX_RX_QUEUES  64
> > 
> > This is different than the 16/8 described above (and seemingly all moot
> > since it becomes a single queue device).
> 
> Nice catch! Those are not even used and are from the earliest days of our driver development. We'll nuke those.

Could you describe the UPT layer a bit?  There were a number of
constants that didn't appear to be used.

> > > +/* interrupt moderation level */
> > > +#define UPT1_IML_NONE     0 /* no interrupt moderation */
> > > +#define UPT1_IML_HIGHEST  7 /* least intr generated */
> > > +#define UPT1_IML_ADAPTIVE 8 /* adpative intr moderation */
> > 
> > enum?  also only appears to support adaptive mode?
> 
> Yes, the Linux driver currently only asks for adaptive mode, but the device supports 8 interrupt moderation levels.
> 
> > > --- /dev/null
> > > +++ b/drivers/net/vmxnet3/vmxnet3_defs.h
> > > +struct Vmxnet3_MiscConf {
> > > +       struct Vmxnet3_DriverInfo driverInfo;
> > > +       uint64_t             uptFeatures;
> > > +       uint64_t             ddPA;         /* driver data PA */
> > > +       uint64_t             queueDescPA;  /* queue descriptor table
> > PA */
> > > +       uint32_t             ddLen;        /* driver data len */
> > > +       uint32_t             queueDescLen; /* queue desc. table len
> > in bytes */
> > > +       uint32_t             mtu;
> > > +       uint16_t             maxNumRxSG;
> > > +       uint8_t              numTxQueues;
> > > +       uint8_t              numRxQueues;
> > > +       uint32_t             reserved[4];
> > > +};
> > 
> > should this be packed (or others that are shared w/ device)?  i assume
> > you've already done 32 vs 64 here
> > 
> 
> No need for packing since the fields are naturally 64-bit aligned. True for all structures shared between the driver and device.

I had quickly looked and thought I saw a hole that would lead to
inconsistent layout for 32-bit vs 64-bit.  I figured I'd be wrong
there ;-)

> > > +#define VMXNET3_MAX_TX_QUEUES  8
> > > +#define VMXNET3_MAX_RX_QUEUES  16
> > 
> > different to UPT, I must've missed some layering here
> 
> These are the authoritiative #defines. Ignore the UPT ones.
> 
> > > --- /dev/null
> > > +++ b/drivers/net/vmxnet3/vmxnet3_drv.c
> > > +       VMXNET3_WRITE_BAR0_REG(adapter, VMXNET3_REG_IMR + intr_idx *
> > 8, 0);
> > 
> > 	writel(0, adapter->hw_addr0 + VMXNET3_REG_IMR + intr_idx * 8)
> > seems just as clear to me.
> 
> Fair enough. We were just trying to clearly show which register accesses go to BAR 0 versus BAR 1.
> 
> > only ever num_intrs=1, so there's some plan to bump this up and make
> > these wrappers useful?
> 
> Yes.
> 
> > > +static void
> > > +vmxnet3_process_events(struct vmxnet3_adapter *adapter)
> > 
> > Should be trivial to break out to it's own MSI-X vector, basically set
> > up to do that already.
> 
> Yes, and the device is configurable to use any vector for any "events", but didn't see any compelling reason to do so. "ECR" events are extremely rare and we've got a shadow copy of the ECR register that avoids an expensive round trip to the device, stored in adapter->shared->ecr. So we can cheaply handle events on the hot Tx/Rx path with minimal overhead. But if you really see a compelling reason to allocate a separate MSI-X vector for events, we can certainly do that.

Nah, just thinking outloud while trying to understand the driver.  I
figured it'd be the + 1 vector (16 + 8 + 1).

thanks,
-chris
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