Message-ID: <4D4ABB67.9070208@siemens.com>
Date: Thu, 03 Feb 2011 15:27:51 +0100
From: Jan Kiszka <jan.kiszka@siemens.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] KVM: Windows 64-bit troubles with user space irqchip
References: <4D496D77.2010405@siemens.com> <4D496FA6.8070301@siemens.com>
	<4D49738D.7080404@redhat.com> <4D4979BD.6080900@siemens.com>
	<20110202154611.GR14984@redhat.com>
	<4D497DAB.7010901@siemens.com> <4D4A64F2.8010309@redhat.com>
	<4D4A7629.1010506@siemens.com> <20110203100407.GA2449@amt.cnet>
	<4D4A7F4B.6050406@siemens.com> <20110203141537.GY14984@redhat.com>
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To: Gleb Natapov <gleb@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, Avi Kivity <avi@redhat.com>, kvm <kvm@vger.kernel.org>, qemu-devel <qemu-devel@nongnu.org>

On 2011-02-03 15:15, Gleb Natapov wrote:
> On Thu, Feb 03, 2011 at 11:11:23AM +0100, Jan Kiszka wrote:
>> On 2011-02-03 11:04, Marcelo Tosatti wrote:
>>> On Thu, Feb 03, 2011 at 10:32:25AM +0100, Jan Kiszka wrote:
>>>> On 2011-02-03 09:18, Avi Kivity wrote:
>>>>> On 02/02/2011 05:52 PM, Jan Kiszka wrote:
>>>>>>>>
>>>>>>>  If there is no problem in the logic of this commit (and I do not see
>>>>>>>  one yet) then we somewhere miss kicking vcpu when interrupt, that should be
>>>>>>>  handled, arrives?
>>>>>>
>>>>>> I'm not yet confident about the logic of the kernel patch: mov to cr8 is
>>>>>> serializing. If the guest raises the tpr and then signals this with a
>>>>>> succeeding, non vm-exiting instruction to the other vcpus, one of those
>>>>>> could inject an interrupt with a higher priority than the previous tpr,
>>>>>> but a lower one than current tpr. QEMU user space would accept this
>>>>>> interrupt - and would likely surprise the guest. Do I miss something?
>>>>>
>>>>> apic_get_interrupt() is only called from the vcpu thread, so it should 
>>>>> see a correct tpr.
>>>>>
>>>>> The only difference I can see with the patch is that we may issue a 
>>>>> spurious cpu_interrupt().  But that shouldn't do anything bad, should it?
>>>>
>>>> I tested this yesterday, and it doesn't confuse Windows. It actually
>>>> receives quite a few spurious IRQs in normal operation, w/ or w/o the
>>>> kernel's tpr optimization.
>>>
>>> http://www.mail-archive.com/kvm@vger.kernel.org/msg41681.html
>>
>> Don't get the scenario yet: We do not inject (or set isr) over the
>> context of apic_set_irq caller.
>>
>>>
>>> tpr of a vcpu should always be inspected in vcpu context, instead of 
>>> iothread context?
>>
>> Maybe this is true for the in-kernel model, but I don't see the issue
>> (anymore) for the way user space works.
>>
> With patch below I can boot Windows7.
> 
> diff --git a/hw/apic.c b/hw/apic.c
> index 146deca..fdcac88 100644
> --- a/hw/apic.c
> +++ b/hw/apic.c
> @@ -600,7 +600,7 @@ int apic_get_interrupt(DeviceState *d)
>      intno = get_highest_priority_int(s->irr);
>      if (intno < 0)
>          return -1;
> -    if (s->tpr && intno <= s->tpr)
> +    if ((s->tpr >> 4) && (intno >> 4) <= (s->tpr >> 4))
>          return s->spurious_vec & 0xff;
>      reset_bit(s->irr, intno);
>      set_bit(s->isr, intno);
> --
> 			Gleb.

Cool, /me too. I would just suggest

diff --git a/hw/apic.c b/hw/apic.c
index 05a115f..13bd7b4 100644
--- a/hw/apic.c
+++ b/hw/apic.c
@@ -582,6 +582,7 @@ int apic_get_interrupt(DeviceState *d)
 {
     APICState *s = DO_UPCAST(APICState, busdev.qdev, d);
     int intno;
+    int tpr;
 
     /* if the APIC is installed or enabled, we let the 8259 handle the
        IRQs */
@@ -594,8 +595,10 @@ int apic_get_interrupt(DeviceState *d)
     intno = get_highest_priority_int(s->irr);
     if (intno < 0)
         return -1;
-    if (s->tpr && intno <= s->tpr)
+    tpr = s->tpr >> 4;
+    if (tpr && (intno >> 4) <= tpr) {
         return s->spurious_vec & 0xff;
+    }
     reset_bit(s->irr, intno);
     set_bit(s->isr, intno);
     apic_update_irq(s);


Unfortunately, that issue was not related to the emulation mode
problems of QEMU.

Thanks!
Jan

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