On 17.07.2013, at 17:19, Yoder Stuart-B08248 wrote: > > >> -----Original Message----- >> From: Alexander Graf [mailto:agraf@xxxxxxx] >> Sent: Wednesday, July 17, 2013 7:19 AM >> To: Gleb Natapov >> Cc: Wood Scott-B07421; Bhushan Bharat-R65777; kvm@xxxxxxxxxxxxxxx; kvm-ppc@xxxxxxxxxxxxxxx; 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
