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

Yoder Stuart-B08248
2013-07-17T15:19:12Z

> -----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?

Stuart