Re: [Qemu-devel] [PATCH v2 1/8] kvm: Set cpu_single_env only once

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-02-11 15:11, Blue Swirl wrote:
> On Sat, Feb 11, 2012 at 14:00, Jan Kiszka <jan.kiszka@web.de> wrote:
>> On 2012-02-11 14:54, Blue Swirl wrote:
>>> On Sat, Feb 11, 2012 at 12:43, Jan Kiszka <jan.kiszka@web.de> wrote:
>>>> On 2012-02-11 12:49, Andreas Färber wrote:
>>>>> Am 11.02.2012 12:25, schrieb Blue Swirl:
>>>>>> I think using cpu_single_env is an indication of a problem, like poor
>>>>>> code, layering violation or poor API (vmport). What is your use case?
>>>>>
>>>>> I couldn't spot any in this series. Jan, note that any new use of env or
>>>>> cpu_single_env will need to be redone when we convert to QOM CPU.
>>>>
>>>> cpu_single_env should have nothing to do with QOM.
>>>>
>>>> The ABIs of vmport and the KVM VAPI require a reference to the calling
>>>> VCPU, and that's why you find tons of them in patch 5.
>>>
>>> Yes, this seems to be another case of a badly designed ABI. I guess
>>> there is no way to change that anymore, just like vmport?
>>
>> Believe me, I grumbled over it more than once while porting it from
>> qemu-kvm. The point is that some (Windows) VMs out there are running
>> already with this option ROM loaded and working this unfortunate ABI.
> 
> Maybe in time those could be deprecated and a ROM using a sane ABI
> introduced instead. After some grace time the old ABI could be finally
> removed.

At some point. But now we have this interface and no other even thought
out. Given that we want to provide a migration path from qemu-kvm to
upstream rather sooner than later, I think there is no way around this
model for a certain, not too short period.

> 
>>>
>>> Some of the cpu_single_env accesses in patch 5 could be avoided when
>>> APIC is moved closer to CPU. VAPIC should be also close to APIC so it
>>> should be able to access the CPU directly. In some other cases the
>>> current state could be passed around instead once it is known.
>>
>> Some callbacks are I/O-port originated, ie. not associated with the
>> per-CPU MMIO area or some MSR. So we would have to pass down the causing
>> CPU to every I/O handler - not sure if that is desired...
> 
> I meant things like vapic_enable_tpr_reporting(), current CPUState
> could be passed via vapic_prepare() easily.

Oh, there is in fact dead code in vapic_enable_tpr_reporting. It just
iterates over all VCPUs, no need to know the current one.

Jan


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