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. 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
