Bill Paul <wp...@windriver.com> writes: > 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.
I figured this would be the case but this is where a comment in the code or commit message would have helped a lot in avoiding future confusion. Regards, Anthony Liguori > > -Bill > >> Tested with windows and linux guests. >> >> Cc: Bill Paul <wp...@windriver.com> >> Reported-by: Yan Vugenfirer <y...@daynix.com> >> Signed-off-by: Michael S. Tsirkin <m...@redhat.com> >> --- >> 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, > wp...@windriver.com | Master of Unix-Fu - Wind River Systems > ============================================================================= > "I put a dollar in a change machine. Nothing changed." - George Carlin > =============================================================================
