Hollis Blanchard wrote: > On Tue, 2007-03-27 at 08:57 +0200, Avi Kivity wrote: > >> Hollis Blanchard wrote: >> >>> Hi Avi, I was wondering what you think is the right abstraction layer to >>> target for porting KVM to non-x86 architectures? To me it looks like >>> libkvm is the answer. >>> >>> The kernel/userland interface is heavily x86-specific, including things >>> like struct kvm_run. So it looks like the higher-level API of >>> kvm_init(), kvm_create(), etc would be the right cut? struct >>> kvm_callbacks is even reasonably portable, especially if cpuid is hidden >>> behind an "arch" callback. >>> >>> >> Disclaimer: I know little about powerpc (or ia64). What I say may or >> may not have any connection with reality. >> >> I don't think we should be aiming at full source portability. >> Virtualization is inherently nonportable, and as it is mostly done in >> hardware, software gets to do the quirky stuff that the hardware people >> couldn't bother with :) instead we should be aiming at code reuse. >> > > I'm not sure I see the distinction you're making. Operating systems > could also be considered "inherently nonportable", yet Linux and the > BSDs support an enormous range of platforms. If you're saying that we > shouldn't try to run x86 MMU code on a PowerPC then I can't agree > more. :) >
No, I'm saying that some #ifdeffery in both libkvm and the ioctl interface is unavoidable. A trivial example is kvm_get_regs(). If you want to do anything other than memcpy() the result, the caller has to be nonportable. kvm_setup_cpuid() doesn't make sense on ppc, as you said. The in*/out* callbacks don't belong, and there will probably be a few callbacks that will leave me puzzled when you add them. The fact is that the "higher level tools" will emulate a powerpc when running on a powerpc, and an x86 when running on an x86. That's different from a webserver which is implementing the http protocol no matter what the underlying platform is. That's what I meant by "inherently nonportable". > Aside from code reuse though (on which I absolutely agree), it's > critical that the interface be the same, i.e. each architecture > implements the same interface in different ways. With that, all the > higher-level tools will work with minimal modification. (This is > analogous to an OS interface like POSIX.) > > A function like sys_read() can be made reasonably portable, but injecting an interrupt into an x86 requires peeking into a register which is aliased to an mmio location (cr8/tpr). No doubt ppc has its own wierdnesses, but they'll be different. Right now this is handled by qemu, which means our higher level tools are _already_ nonportable. >> I think there's some potential there: >> >> - memory slot management, including the dirty log, could be mostly >> reused (possibly updated for multiple page sizes). possibly msrs as well. >> > > I'm not familiar with KVM's memory slots or dirty log. My first > impression was that the dirty log is tied to the x86 shadow pagetable > implementation, but I admit I haven't investigated further. > The implementation is, but the interface and use is generic. The dirty log is used for two purposes: - minimization of screen updates on framebuffer changes - tracking pages which need to be re-copied during live migration Hopefully the interface and some parts of the kernel code can be reused. The memory slots thing is just a way for userspace to specify physically discontiguous memory. Each slot is contiguous within itself, but different slots may be discontiguous. It is used for the framebuffer, and for various memory holes in x86 (640KB-1MB and the pci hole). > >> I don't see a big difference between the ioctl layer and libkvm. In >> general, a libkvm function is an ioctl, and kvm_callback members are a >> decoding of kvm_run fields. If you edit kvm_run to suit your needs, you >> can probably reuse some of it. >> > > kvm_run as it stands is 100% x86-specific. (I doubt it could even be > easily adapted for ia64, which is more similar to x86 than PowerPC.) So > right now the kernel ioctl interface has an architecture-specific > component, which violates the principle of identical interfaces I > described earlier. > Just #ifdef the x86 specific parts away, and add your own magic where necessary. > That means we either a) need to change the kernel interface or b) define > a higher-level interface that *is* identical. That higher-level > interface would be libkvm, hence my original question. > > Does my original question make more sense now? If you make libkvm the > official interface, you would at least need to hide the "cpuid" > callback, since it is intimately tied to an x86 instruction. > Well, libkvm is _an_ official interface. Any changes needed to make it portable are welcome. [I have a feeling we're talking a little past each other, probably due to me not knowing ppc at any level of detail. No doubt things will become clearer when the code arrives] -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
