Nadav Amit <nadav.amit@xxxxxxxxx> writes:
> Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote:
>
>> On 2015-04-08 19:40, Nadav Amit wrote:
>>> 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”).
>>
>> Ah. No problem. My brain apparently fixed that typo up unnoticed.
>>
>>>> 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)?
>>
>> No, I don't think there is a need to leave this inaccurate for QEMU if
>> our included BIOS gets it right. I don't know what the backward
>> bug-compatibility of KVM is, though. Maybe you can identify since when
>> our BIOS is fine so that we can discuss time frames.
>
> I think that it was addressed in commit
> 19c1a7692bf65fc40e56f93ad00cc3eefaad22a4 ("Initialize the LINT LVTs on the
> local APIC of the BSP.”) So it should be included in seabios 0.5.0, which
> means qemu 0.12 - so we are talking about the end of 2009 or start of 2010.
The probability that someone will use a newer version of kernel with something
as old as 0.12 is probably minimal. I think it's ok to change it with a comment
indicating the reason. To be on the safe side, however, a user changeable switch
is something worth considering.
> What is the verdict?
>
> Nadav--
> To unsubscribe from this list: send the line "unsubscribe kvm" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
