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 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.

> 
> 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...

Jan


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