Re: [PATCH 3/5] booke: define reset and shutdown hcalls

Alexander Graf
2013-07-17T15:21:19Z

On 17.07.2013, at 17:19, Yoder Stuart-B08248 wrote:

>
>
>> -----Original Message-----
>> From: Alexander Graf [mailto:agraf@suse.de]
>> Sent: Wednesday, July 17, 2013 7:19 AM
>> To: Gleb Natapov
>> Cc: Wood Scott-B07421; Bhushan Bharat-R65777; kvm@vger.kernel.org; kvm-ppc@vger.kernel.org; Yoder
>> Stuart-B08248; Bhushan Bharat-R65777
>> Subject: Re: [PATCH 3/5] booke: define reset and shutdown hcalls
>>
>>
>> On 17.07.2013, at 13:00, Gleb Natapov wrote:
>>
>>> On Tue, Jul 16, 2013 at 06:04:34PM -0500, Scott Wood wrote:
>>>> On 07/16/2013 01:35:55 AM, Gleb Natapov wrote:
>>>>> On Mon, Jul 15, 2013 at 01:17:33PM -0500, Scott Wood wrote:
>>>>>> On 07/15/2013 06:30:20 AM, Gleb Natapov wrote:
>>>>>>> There is no much sense to share hypercalls between architectures.
>>>>>>> There
>>>>>>> is zero probability x86 will implement those for instance
>>>>>>
>>>>>> This is similar to the question of whether to keep device API
>>>>>> enumerations per-architecture... It costs very little to keep it in
>>>>>> a common place, and it's hard to go back in the other direction if
>>>>>> we later realize there are things that should be shared.
>>>>>>
>>>>> This is different from device API since with device API all arches
>>>>> have
>>>>> to create/destroy devices, so it make sense to put device lifecycle
>>>>> management into the common code, and device API has single entry point
>>>>> to the code - device fd ioctl - where it makes sense to handle common
>>>>> tasks, if any, and despatch others to specific device implementation.
>>>>>
>>>>> This is totally unlike hypercalls which are, by definition, very
>>>>> architecture specific (the way they are triggered, the way parameter
>>>>> are passed from guest to host, what hypercalls arch needs...).
>>>>
>>>> The ABI is architecture specific. The API doesn't need to be, any
>>>> more than it does with syscalls (I consider the
>>>> architecture-specific definition of syscall numbers and similar
>>>> constants in Linux to be unfortunate, especially for tools such as
>>>> strace or QEMU's linux-user emulation).
>>>>
>>> Unlike syscalls different arches have very different ideas what
>>> hypercalls they need to implement, so while with unified syscall space I
>>> can see how it may benefit (very) small number of tools, I do not see
>>> what advantage it will give us. The disadvantage is one more global name
>>> space to manage.
>>>
>>>>>> Keeping it in a common place also makes it more visible to people
>>>>>> looking to add new hcalls, which could cut down on reinventing the
>>>>>> wheel.
>>>>> I do not want other arches to start using hypercalls in the way
>>>>> powerpc
>>>>> started to use them: separate device io space, so it is better to hide
>>>>> this as far away from common code as possible :) But on a more serious
>>>>> note hypercalls should be a last resort and added only when no other
>>>>> possibility exists, so people should not look what hcalls others
>>>>> implemented, so they can add them to their favorite arch, but they
>>>>> should have a problem at hand that they cannot solve without
>>>>> hcall, but
>>>>> at this point they will have pretty good idea what this hcall
>>>>> should do.
>>>>
>>>> Why are hcalls such a bad thing?
>>>>
>>> Because they often used to do non architectural things making OSes
>>> behave different from how they runs on real HW and real HW is what
>>> OSes are designed and tested for. Example: there once was a KVM (XEN
>>> have/had similar one) hypercall to accelerate MMU operation. One thing it
>>> allowed is to to flush tlb without doing IPI if vcpu is not running. Later
>>> optimization was added to Linux MMU code that _relies_ on those IPIs for
>>> synchronisation. Good that at that point those hypercalls were already
>>> deprecated on KVM (IIRC XEN was broke for some time in that regard). Which
>>> brings me to another point: they often get obsoleted by code improvement
>>> and HW advancement (happened to aforementioned MMU hypercalls), but they
>>> hard to deprecate if hypervisor supports live migration, without live
>>> migration it is less of a problem. Next point is that people often try
>>> to use them instead of emulate PV or real device just because they
>>> think it is easier, but it is often not so. Example: pvpanic device was
>>> initially proposed as hypercall, so lets say we would implement it as
>>> such. It would have been KVM specific, implementation would touch core
>>> guest KVM code and would have been Linux guest specific. Instead it was
>>> implemented as platform device with very small platform driver confined
>>> in drivers/ directory, immediately usable by XEN and QEMU tcg in addition
>>
>> This is actually a very good point. How do we support reboot and shutdown for TCG guests? We surely
>> don't want to expose TCG as KVM hypervisor.
>
> Hmm...so are you proposing that we abandon the current approach,
> and switch to a device-based mechanism for reboot/shutdown?

Reading Gleb's email it sounds like the more future proof approach, yes. I'm not quite sure yet where we should plug this though.

Alex