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Re: [Qemu-devel] [PATCH] e1000e: Don't zero out buffer address in rx des

From: Dmitry Fleytman
Subject: Re: [Qemu-devel] [PATCH] e1000e: Don't zero out buffer address in rx descriptor
Date: Wed, 19 Oct 2016 10:57:39 +0300

> On 19 Oct 2016, at 10:25 AM, Kevin Wolf <address@hidden> wrote:
> Am 19.10.2016 um 08:48 hat Dmitry Fleytman geschrieben:
>>    Another related thing that I noticed while debugging this and turning on
>>    tracing is that the interrupt throttling timers kept firing even if
>>    there was no activity at all. Something might be wrong, there, too.
>>    Next thing I wondered why throttling was enabled at all because the spec
>>    says the default is 0 (turned off). So one thing that I'm pretty sure is
>>    just a misunderstanding is the following defintion:
>>    #define E1000E_MIN_XITR     (500) /* No more then 7813 interrupts per
>>                                        second according to spec */
>>    As I understand it, the spec is just giving an example there and lower
>>    values are valid as well. At the very least, 0 should be accepted as a
>>    special case because it means "disabled" and it's specified to be the
>>    default.
>> Right, this according to the spec this value should be 0 by default and
>> throttling should be disabled.
>> Current device implementation does not allow specification of
>> throttling interval less than 500 and treats interval 0 as throttling
>> enabled with interval 500.
>> This is done by intention because according to the spec (
>> device cannot produce more than 7813 interrupts per second even when
>> throttling is disabled. Therefore, even in case of interrupt storm
>> (continuous interrupt re-injection by device), number of interrupts
>> produced by device is limited and CPU (driver) has enough time to do
>> its job and handle problematic interrupt state.
> I think you're misinterpreting the spec here. This is the paragraph
> we're talking about, right?
>    For example, if the interval is programmed to 500 (decimal), the
>    82574 guarantees the CPU is not interrupted by it for 128 µs from
>    the last interrupt. The maximum observable interrupt rate from the
>    82574 should never exceed 7813 interrupts/sec.
> It says "for example", so this is just demonstrating how you can
> calculate the effects of a specific throttling setting. It says that
> _if_ you set ITR to 500, you get an interrupt at most every
> 500 * 256 ns = 128 µs. And 1 / 128 µs = 7821.5 Hz, so this is the
> effective maximum frequency that _this specific_ ITR setting allows.
> I also don't think it would make any sense for hardware to be unable to
> trigger interrupts more often than that. Triggering an interrupt is not
> a complex operation that involves a lot of calculation or anything.

Hi Kevin,

Yes, I assume that sentence

“The maximum observable interrupt rate from the
82574 should never exceed 7813 interrupts/sec."

is not a related to a specific case, but describes a generic limitation,
however it might be I’m misreading the spec indeed.

>> Opposed to this, virtual device is able to raise interrupts with rate
>> limited by CPU speed only therefore driver has no chance to fix
>> interrupt storm condition. 
>> Windows e1000e drivers rely on upper limit for number of interrupts
>> per second in some cases and absence of this limit leads to infinite
>> interrupt storms.
>> To summarise, while usage of throttling mechanisms is a little bit
>> different from what specification says, effective emulated device
>> behavior is totally compliant to the real device.
> So Windows doesn't configure ITR (i.e. it is 0) even though it can't
> handle unlimited interrupts? That would be a driver bug then, and
> perhaps an important enough one to keep a workaround in our code. But
> then let's be explicit that this is a workaround for a Windows bug and
> not mandated by the spec.
> I'm not sure in what setup you produced this error, but possibly a
> reason why this doesn't happen with real hardware isn't the NIC itself
> but the backend: Communication with the host can obviously be faster
> than talking to a physical network (so if you were doing the latter, the
> rate in the VM wouldn't be limited by the CPU, but by the physical
> network).

This issue is reproduced on device disable and not related 
to intensive device/backend communication. One RX packet with
right timing is enough to trigged the problem.

The same issue was fixed in e1000 device some time ago as well.


>>    As this wasn't blocking me after I had patched the above constant
>>    locally to 0 so that I could see the actually meaningful trace events, I
>>    didn't dig deeper, but I suspect that my local workaround for the
>>    trace point spam may actually be a valid fix.
>> This part that looks suspicious.
>> Even when throttling is enabled, timers should be idle
>> unless there are packets on corresponding path.
>> In case you see times firing when device is idle - something
>> might be wrong in throttling mechanisms.
> I saw a lot of trace events related to the timer between the really
> interesting information (that is, multiple timer invocations with
> nothing else in between), but I think it may not have been always. I
> didn't really look too closely at the cause of this one because it was
> easy enough to patch it out of the way.
> Kevin

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