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

Bhushan Bharat-R65777
2013-07-17T15:59:30Z

> -----Original Message-----
> From: Alexander Graf [mailto:agraf@suse.de]
> Sent: Wednesday, July 17, 2013 9:22 PM
> To: Bhushan Bharat-R65777
> Cc: Yoder Stuart-B08248; Wood Scott-B07421; kvm@vger.kernel.org; kvm-
> ppc@vger.kernel.org; Gleb Natapov
> Subject: Re: [PATCH 3/5] booke: define reset and shutdown hcalls
>
>
> 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.
>

Ah, so we can choose any address range in ccsr space of a PV machine (-M ppce500 / e500plat). What about MPC8544DS machine?.

So what is preferred way, vitio-reset/shutdown device or the above mentioned ?

Thanks
-Bharat