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

Alexander Graf
2013-07-17T15:52:09Z

On 17.07.2013, at 17:47, Bhushan Bharat-R65777 wrote:

>
>>>>>> 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.
>>>
>>> What do you mean...where the paravirt device would go in the physical
>>> address map??
>>
>> Right. Either we
>>
>> - let the guest decide (PCI)
>> - let QEMU decide, but potentially break the SoC layout (SysBus)
>> - let QEMU decide, but only for the virt machine so that we don't break anyone
>> (PlatBus)
>
> Can you please elaborate above two points ?

If we emulate an MPC8544DS, we need to emulate an MPC8544DS. Any time we diverge from the layout of the original chip, things can break.

However, for our PV machine (-M ppce500 / e500plat) we don't care about real hardware layouts. We simply emulate a machine that is 100% described through the device tree. So guests that can't deal with the machine looking different from real hardware don't really matter anyways, since they'd already be broken.

Alex