Re: [dpdk-dev] [PATCH 1/4] vhost: inroduce operation to get vDPA queue stats

[Shahaf Shuler <shahafs@mellanox.com>]

Thursday, April 16, 2020 4:20 PM, Maxime Coquelin:
> Subject: Re: [PATCH 1/4] vhost: inroduce operation to get vDPA queue stats
> 
> Hi Matan,
> 
> On 4/16/20 11:06 AM, Matan Azrad wrote:
> > Hi Maxime
> >
> > Can you point on specific vendor specific counter I suggested?
> 
> No, I can't, but I think we can expect that other vendors may have other
> counters they would be interested to dump.
> 
> Maybe Intel has some counters in the IFC that they could dump.
> Xiao, any thoughts?
> 
> > I think all of them come directly from virtio protocols.
> 
> exceed_max_chain, for example. Doesn't the spec specify that a descriptors
> chain can be as long as the size of the virtqueue?
> 
> Here it seems to indicate the device could support less.

Spec allows device to limit the max supported chain (see [1]). 

> 
> Also, as the spec evolves, we may have new counters that comes up, so
> better to have something flexible from the start IMHO to avoid ABI
> breakages.

I think there are better ways to address that, e.g.:
1. have some reserved fields for future
2. have the option to point to next item, and by that link chain of stat structures

> 
> Maybe we can have some common xstats names for the Virtio related
> counters define in vdpa lib, and then the vendors can specify more vendor-
> specific counters if they wish?

xstats are good, and we should have it to expose the vendor specific counters. The basic counters though, should be simple and vendor agnostic so that any SW/scripting layer on top of the DPDK can easily use and expose it. 
Hence I think it will be good to have the basic counters with well-defined stats structure as part of the vdpa stats API. Is the exceed_max_chain is the only counter you find odd or there are more? 

> 
> Thanks,
> Maxime

[1]
https://github.com/oasis-tcs/virtio-spec/blob/master/packed-ring.tex#L498