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

Yoder Stuart-B08248
2013-07-17T15:37:38Z

> -----Original Message-----
> From: Alexander Graf [mailto:agraf@suse.de]
> Sent: Wednesday, July 17, 2013 10:21 AM
> To: Yoder Stuart-B08248
> Cc: Wood Scott-B07421; Bhushan Bharat-R65777; 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 17:19, 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.

What do you mean...where the paravirt device would go in the physical
address map??

Stuart