Bandan Das <bsd@xxxxxxxxxx> wrote:
> 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.
I don’t see any existing mechanism for KVM to be aware of its user type and
version. I do see another case of KVM hacks that are intended for fixing
very old QEMU bugs (see 3a624e29c75 changes in vmx_set_segment, which are
from pretty much the same time-frame of the issue I try to fix).
Since this is something which would follow around, please advise what would
be the format. A new ioctl that would supply the userspace “type” (according
to predefined constants) and version?
Thanks,
Nadav--
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