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Re: [Qemu-devel] [PATCH] e1000e: Don't zero out buffer address in rx des
Re: [Qemu-devel] [PATCH] e1000e: Don't zero out buffer address in rx descriptor
Wed, 19 Oct 2016 12:07:40 +0200
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
> 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
> second according to spec
> 10.2.4.2 */
> As I understand it, the spec is just giving an example there and
> values are valid as well. At the very least, 0 should be accepted
> special case because it means "disabled" and it's specified to be
> Right, this according to the spec this value should be 0 by default
> 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 (10.2.4.2)
> 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
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.
What we're doing may still have merit, as a workaround for a guest
> 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
> 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.
Re: [Qemu-devel] [PATCH] e1000e: Don't zero out buffer address in rx descriptor, Jason Wang, 2016/10/20