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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 13:15:24 +0300

> On 19 Oct 2016, at 13:07 PM, Kevin Wolf <address@hidden> wrote:
> Am 19.10.2016 um 09:57 hat Dmitry Fleytman geschrieben:
>>    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.
> For me everything hints at this being only an example: Not only do the
> numbers match the example made in the previous sentence (which is
> explicitly called an example) and look weird as a real limit, but it's
> also in the same paragraph as the explicit example and the spec is
> generally good at starting a new paragraph when talking about a new
> aspect.

I tend to agree.

> I don't care enough to actually make you change anything, but I wanted
> you to be aware that the interpretation of the spec as coded into our
> emulation isn't clear at all (in fact, I would think it's clear that
> it's _not_ meant this way) and that real hardware probably doesn't do
> the same thing as we do.

Thanks, Kevin.

> What we're doing may still have merit, as a workaround for a guest
> driver bug.
>>        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.
> Commit 9596ef7c was good in flagging it as a guest driver bug. Only a
> later series brought in the questionable spec interpretation.
> Kevin

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