Re: [PATCH] KVM: x86: ioapic: Clear IRR for rtc bit when rtc EOI gotten

[Paolo Bonzini]

On 14/12/2017 14:01, Liran Alon wrote:
>> But in our test, we found that there is a possible situation that Vcpu
>> fails to read
>> RTC_REG_C to clear irq, This could happens while two VCpus are
>> writing/reading
>> registers at the same time, for example, vcpu 0 is trying to read
>> RTC_REG_C,
>> so it write RTC_REG_C first, where the s->cmos_index will be RTC_REG_C,
>> but before it tries to read register C, another vcpu1 is going to read
>> RTC_YEAR,
>> it changes s->cmos_index to RTC_YEAR by a writing action.
>> The next operation of vcpu0 will be lead to read RTC_YEAR, In this
>> case, we will miss
>> calling qemu_irq_lower(s->irq) to clear the irq. After this, kvm will
>> never inject RTC irq,
>> and Windows VM will hang.
> 
> If I understood correctly, this looks to me like a race-condition bug in
> the Windows guest kernel. In real-hardware this race-condition will also
> cause the RTC_YEAR to be read instead of RTC_REG_C.
> Guest kernel should make sure that 2 CPUs does not attempt to read a
> CMOS register in parallel as they can override each other's cmos_index.
> 
> See for example how Linux kernel makes sure to avoid such kind of issues
> in rtc_cmos_read() (arch/x86/kernel/rtc.c) by grabbing a cmos_lock.

Lei and I looked at it further, and the root cause is not the missed EOI
in QEMU.  Rather it's a bug in ioapic.c's tracking of RTC interrupts.

Paolo