> -----Original Message----- > From: Alexander Graf [mailto:agraf@xxxxxxx] > Sent: Wednesday, July 17, 2013 10:21 AM > To: Yoder Stuart-B08248 > Cc: Wood Scott-B07421; Bhushan Bharat-R65777; kvm@xxxxxxxxxxxxxxx; kvm-ppc@xxxxxxxxxxxxxxx; 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@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. What do you mean...where the paravirt device would go in the physical address map?? 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