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Re: [Qemu-devel] [PATCH] e1000: make ICS write-only
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH] e1000: make ICS write-only |
Date: |
Wed, 9 Jan 2013 23:44:38 +0200 |
On Wed, Jan 09, 2013 at 09:30:43AM -0800, Bill Paul wrote:
> Of all the gin joints in all the towns in all the world, Michael S. Tsirkin
> had to walk into mine at 02:51:00 on Wednesday 09 January 2013 and say:
>
> > Since commit b1332393cdd7d023de8f1f8aa136ee7866a18968,
> > qemu started updating ICS register when interrupt
> > is sent, with the intent to match spec better
> > (guests do not actually read this register).
> > However, the function set_interrupt_cause where ICS
> > is updated is often called internally by
> > device emulation so reading it does not produce the last value
> > written by driver. Looking closer at the spec,
> > it documents ICS as write-only, so there's no need
> > to update it at all. I conclude that while harmless this line is useless
> > code so removing it is a bit cleaner than keeping it in.
>
> You are wrong.
>
> I know what the spec says. The spec lies (or at the very least, it doesn't
> agree with reality). With actual PRO/1000 hardware, the ICS register is
> readable. It provides a way to obtain the currently pending interrupt bits
> without the auto-clear behavior of the ICR register (which in some cases is
> actually detrimental).
>
> The Intel 10GbE NICs (82598, 82599, x540) are the same way (they're very
> similar in design to the PRO/1000s, particularly the 82575, 82576, 82580 and
> later devices). The spes for the 10GbE devices _also_ say that ICS is read
> only. But if you look at the Intel drivers for those chips, you'll see they
> have actually implemented a workaround for a device errata (I think the for
> the 82598) that actually requires reading the ICS register. If you had
> implemented a PRO/10GbE virtual device based on the same chip and obeyed the
> spec the same way, I suspect Intel's driver would break.
>
> I actually have in my possession real PRO/1000 silicon going all the way back
> to the 82543 and have tested every single one of them, and they all work like
> this. I've also asked the Intel LAN access division people about it and they
> said that in spite of it not being documented as readable, there's nothing
> particularly wrong with doing this.
>
> The problem with using ICR is that the auto-clear behavior can sometimes
> result in some awkward interrupt handling code. You need to test ICR in
> interrupt context to see if there are events pending, and then schedule some
> other thread to handle them. But since reading ICR clears the bits, you need
> to save the events somewhere so that the other thread knows what events need
> servicing. Keeping the saved copy of pending events properly synchronized so
> that you don't lose any is actually big challenge. The VxWorks PRO/1000
> driver
> used to have some very ugly code in it to deal with it, all of which became
> much simpler when using the ICS register instead.
>
> I know what the spec says. But this is a case where the spec only says that
> because Intel decided not to disclose what the hardware actually does. They
> also don't tell you about all the hidden debug registers in the hardware
> either, but that doesn't mean they don't exist.
>
> So pretty please, with sugar on top, leave this code alone.
>
> -Bill
OK now since there's no spec, I'd like to find out how does the
register behave. Let's assume I read ICR. This clears ICR - does it
also clear ICS?
Thanks,
MST
> > Tested with windows and linux guests.
> >
> > Cc: Bill Paul <address@hidden>
> > Reported-by: Yan Vugenfirer <address@hidden>
> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > ---
> > hw/e1000.c | 1 -
> > 1 file changed, 1 deletion(-)
> >
> > diff --git a/hw/e1000.c b/hw/e1000.c
> > index 92fb00a..928d804 100644
> > --- a/hw/e1000.c
> > +++ b/hw/e1000.c
> > @@ -230,7 +230,6 @@ set_interrupt_cause(E1000State *s, int index, uint32_t
> > val) val |= E1000_ICR_INT_ASSERTED;
> > }
> > s->mac_reg[ICR] = val;
> > - s->mac_reg[ICS] = val;
> > qemu_set_irq(s->dev.irq[0], (s->mac_reg[IMS] & s->mac_reg[ICR]) != 0);
> > }
>
> --
> =============================================================================
> -Bill Paul (510) 749-2329 | Member of Technical Staff,
> address@hidden | Master of Unix-Fu - Wind River Systems
> =============================================================================
> "I put a dollar in a change machine. Nothing changed." - George Carlin
> =============================================================================
Re: [Qemu-devel] [PATCH] e1000: make ICS write-only,
Michael S. Tsirkin <=