Re: x86: Question regarding the reset value of LINT0

[Avi Kivity <avi.kivity@gmail.com>]

On 04/09/2015 09:21 PM, Nadav Amit wrote:
> Bandan Das <bsd@redhat.com> wrote:
>
>> Nadav Amit <nadav.amit@gmail.com> writes:
>>
>>> Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>>
>>>> On 2015-04-08 19:40, Nadav Amit wrote:
>>>>> Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>>>>
>>>>>> On 2015-04-08 18:59, Nadav Amit wrote:
>>>>>>> Jan Kiszka <jan.kiszka@siemens.com> 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?

That would be madness.  KVM shouldn't even know that qemu exists, let 
alone track its versions.

Simply add a new toggle KVM_USE_STANDARD_LAPIC_LVT_INIT and document 
that userspace MUST use it.  Old userspace won't, and will get the old 
buggy behavior.