netdev - Re: [net-next 10/11] net/mlx5e: kTLS, Add kTLS RX resync support

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Date:   Fri, 29 May 2020 22:47:14 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     Boris Pismenny <borisp@...lanox.com>,
        "kuba@...nel.org" <kuba@...nel.org>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [net-next 10/11] net/mlx5e: kTLS, Add kTLS RX resync support

On Fri, 2020-05-29 at 14:50 -0700, Jakub Kicinski wrote:
> On Fri, 29 May 2020 20:44:29 +0000 Saeed Mahameed wrote:
> > > I thought you said that resync requests are guaranteed to never
> > > fail?
> > 
> > I didn't say that :),  maybe tariq did say this before my review,
> 
> Boris ;)
> 
> > but basically with the current mlx5 arch, it is impossible to
> > guarantee
> > this unless we open 1 service queue per ktls offloads and that is
> > going
> > to be an overkill!
> 
> IIUC every ooo packet causes a resync request in your implementation
> -
> is that true?
> 

For tx yes, for RX i am not sure, this is a hw flow that I am not fully
familiar with.

Anyway according to Tariq, The hw might generate more than one resync
request on the same flow, and this is all being handled by the driver
correctly. I am not sure if this is what you are looking for.

Maybe Tariq/Boris can elaborate more on the hw resync mechanism.

> It'd be great to have more information about the operation of the
> device in the commit message..
> 

How about:

Resync flow occurs when packets have been lost and the device lost
track of TLS records. The device attempts to resync by tracking TLS
records, and sends a resync request to driver. The TLS Progress Params
Context holds the TCP-SN of the record where the device began tracking
records and counting them. The driver will acknowledge the TCP-SN if it
matches a legal record by setting the TLS Static Params Context.

? 
we can elaborate more with a step by step procedure.. if you think it
is required.

> > This is a rare corner case anyway, where more than 1k tcp
> > connections
> > sharing the same RX ring will request resync at the same exact
> > moment. 
> 
> IDK about that. Certain applications are architected for max
> capacity,
> not efficiency under steady load. So it matters a lot how the system
> behaves under stress. What if this is the chain of events:
> 
> overload -> drops -> TLS steams go out of sync -> all try to resync
> 
> We don't want to add extra load on every record if HW offload is
> enabled. That's why the next record hint backs off, checks socket 
> state etc.
> 
> BTW I also don't understand why mlx5e_ktls_rx_resync() has a
> tls_offload_rx_force_resync_request(sk) at the end. If the update 
> from the NIC comes with a later seq than current, request the sync 
> for _that_ seq. I don't understand the need to force a call back on
> every record here. 

Good point theoretically should work, unless we have some limitations
that i am not seeing, i will let Tariq comment on this.

> 
> Also if the sync failed because queue was full, I don't see how
> forcing 
> another sync attempt for the next record is going to match?

In this case i guess we need to abort and wait for the hw to issue anew resync request ..