> -----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? Stuart -- 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
