Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote:
> On 2015-04-08 18:59, Nadav Amit wrote:
>> Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote:
>>
>>> On 2015-04-08 18:40, Nadav Amit wrote:
>>>> Hi,
>>>>
>>>> I would appreciate if someone explains the reason for enabling LINT0 during
>>>> APIC reset. This does not correspond with Intel SDM Figure 10-8: “Local
>>>> Vector Table” that says all LVT registers are reset to 0x10000.
>>>>
>>>> In kvm_lapic_reset, I see:
>>>>
>>>> apic_set_reg(apic, APIC_LVT0,
>>>> SET_APIC_DELIVERY_MODE(0, APIC_MODE_EXTINT));
>>>>
>>>> Which is actually pretty similar to QEMU’s apic_reset_common:
>>>>
>>>> if (bsp) {
>>>> /*
>>>> * LINT0 delivery mode on CPU #0 is set to ExtInt at initialization
>>>> * time typically by BIOS, so PIC interrupt can be delivered to the
>>>> * processor when local APIC is enabled.
>>>> */
>>>> s->lvt[APIC_LVT_LINT0] = 0x700;
>>>> }
>>>>
>>>> Yet, in both cases, I miss the point - if it is typically done by the BIOS,
>>>> why does QEMU or KVM enable it?
>>>>
>>>> BTW: KVM seems to run fine without it, and I think setting it causes me
>>>> problems in certain cases.
>>>
>>> I suspect it has some historic BIOS backgrounds. Already tried to find
>>> more information in the git logs of both code bases? Or something that
>>> indicates of SeaBIOS or BochsBIOS once didn't do this initialization?
>> Thanks. I found no indication of such thing.
>>
>> QEMU’s commit message (0e21e12bb311c4c1095d0269dc2ef81196ccb60a) says:
>>
>> Don't route PIC interrupts through the local APIC if the local APIC
>> config says so. By Ari Kivity.
>>
>> Maybe Avi Kivity knows this guy.
>
> ths? That should have been Thiemo Seufer (IIRC), but he just committed
> the code back then (and is no longer with us, sadly).
Oh… I am sorry - I didn’t know about that.. (I tried to make an unfunny joke
about Avi knowing “Ari”).
> But if that commit went in without any BIOS changes around it, QEMU
> simply had to do the job of the latter to keep things working.
So should I leave it as is? Can I at least disable in KVM during INIT (and
leave it as is for RESET)?
Thanks,
Nadav
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