Re: [Qemu-devel] Re: [RFT][PATCH 07/15] qemu_irq: Add IRQ handlers with delivery feedback

[Gleb Natapov]

On Sun, May 30, 2010 at 12:10:16PM +0000, Blue Swirl wrote:
> >> >> You missed the key word 'stopped'. If the timer is really stopped, no
> >> >> IRQs should ever come out afterwards, just like on real HW. For the
> >> >> emulation, this means loss of ticks which should have been delivered
> >> >> before the change.
> >> >>
> >> > I haven't missed it. I describe to you reality of the situation. You want
> >> > to change reality to be more close to what you want it to be by adding
> >> > words to my description.
> >>
> >> Quoting Jan: 'So what to do with the backlog when the timer is
> >> stopped?' I didn't add any words to your description, please be more
> >> careful with your attributions. Why do you think I want to change the
> >> reality?
> > Please refer to my words when you answer to my quote. You quoted my
> > answer to you statement:
> >  Gleb only mentioned the frequency change, I thought that was not so big
> >  problem. But I don't think this case should be allowed happen at all,
> >  it can't exist on real HW.
> 
> With 'this case' I was referring to 'case with timer stopped', not
> 'case which Gleb mentioned'.
> 
> > No 'stopped' was under discussion nowhere.
> 
> It's clearly written there in the sentence Jan wrote.
> 
Jan, not me, but lets leave this topic alone since you agree that
stopped is just a case of frequency change anyway.

> > FWIW 'stopped' is just a case
> > of frequency change.
> 
> True.
> 
> >
> >>
> >> XP frequency change isn't the same case as timer being stopped.
> >>
> > And what is the big difference exactly?
> 
> Because after the timer is stopped, its extremely unrealistic to send
> any IRQs. Whereas if the frequency is changed to some other nonzero
> value, we can cheat and inject some more queued IRQs.
> 
Correct, when gets disables clock source (by reset or any other means)
coalesced backlog should be forgotten.

> Anyway, if this case is not interesting because it doesn't happen in
> real life emulation scenarios, we can forget it no matter how buggy
> the current QEMU implementation is.
> 
> >> > Please just go write code, experiment, debug
> >> > and _then_ come here with design.
> >>
> >> I added some debugging to RTC, PIC and APIC. I also built a small
> >> guest in x86 assembly to test the coalescing. However, in the tests
> >> with this guest and others I noticed that the coalescing only happens
> >> in some obscure conditions.
> > So try with real guest and with real load.
> 
> Well, I'd like to get the test program also trigger it. Now I'm getting:
> apic: write: 00000350 = 00000000
> apic: apic_reset_irq_delivered: old coalescing 0
> apic: apic_local_deliver: vector 3 delivery mode 0
> apic: apic_set_irq: coalescing 1
> apic: apic_get_irq_delivered: returning coalescing 1
> apic: apic_reset_irq_delivered: old coalescing 1
> apic: apic_local_deliver: vector 3 delivery mode 0
> apic: apic_set_irq: coalescing 0
> apic: apic_get_irq_delivered: returning coalescing 0
> apic: apic_reset_irq_delivered: old coalescing 0
> apic: apic_local_deliver: vector 3 delivery mode 0
> apic: apic_set_irq: coalescing 0
> 
> It looks like some other IRQs cause the coalescing, because also
> looking at RTC code, it seems it's not possible for RTC to raise the
> IRQ (except update IRQ, alarm etc.) without calling
> apic_reset_irq_delivered().
> 
> I've attached my test program. Compile:
> gcc -m32 -o coalescing coalescing.S -ffreestanding -nostdlib -Wl,-T
> coalescing.ld -g && objcopy -Obinary coalescing coalescing.bin
> 
> Run:
> qemu -L . -bios coalescing.bin -no-hpet -rtc-td-hack
> 
The application does not work for me. Looks like it fails to enter
protected mode. $pc jumps from 0x00000000fffffff0 to 0x00000000000f003e
and back.

> >>
> >> By default the APIC's delivery method for IRQs is ExtInt and
> >> coalescing counting happens only with Fixed. This means that the guest
> >> needs to reprogram APIC. It also looks like RTC interrupts need to be
> >> triggered. But I didn't see both of these to happen simultaneously in
> >> my tests with Linux and Windows guests. Of course, -rtc-td-hack flag
> >> must be used and I also disabled HPET to be sure that RTC would be
> >> used.
> >>
> >> With DEBUG_COALESCING enabled, I just get increasing numbers for
> >> apic_irq_delivered:
> >> apic: apic_set_irq: coalescing 67123
> >> apic: apic_set_irq: coalescing 67124
> >> apic: apic_set_irq: coalescing 67125
> > So have you actually used -rtc-td-hack option? I compiled head of
> > qemu.git with DEBUG_COALESCING and run WindowsXP guest with -rtc-td-hack
> > and I get:
> > apic: apic_reset_irq_delivered: old coalescing 3
> > apic: apic_set_irq: coalescing 1
> > apic: apic_get_irq_delivered: returning coalescing 1
> > apic: apic_set_irq: coalescing 2
> > apic: apic_set_irq: coalescing 3
> > apic: apic_set_irq: coalescing 4
> > apic: apic_set_irq: coalescing 5
> > apic: apic_set_irq: coalescing 6
> > apic: apic_reset_irq_delivered: old coalescing 6
> > apic: apic_set_irq: coalescing 1
> > apic: apic_get_irq_delivered: returning coalescing 1
> >
> >>
> >> If the hack were active, the numbers would be close to zero (or at
> >> least some point) because apic_reset_irq_delivered would be called,
> >> but this does not happen. Could you specify a clear test case with
> >> which the coalescing action could be tested? Linux or BSD based,
> >> please.
> > Linux don't use RTC as time source and I don't know about BSD, so no
> > Linux or BSD test case for you, sorry. Run WindowXP standard HAL and put
> > heavy load on the host. You can run video inside the gust to trigger
> > coalescing more easily.
> 
> I don't have Windows XP, sorry.
> 
Will be hard to debug Windows time drift without Windows ;) Do you know
what time source BSD uses?

--
                        Gleb.