Am 27.08.2014 18:08, schrieb Eduardo Habkost: > On Wed, Aug 27, 2014 at 05:58:49PM +0200, Andreas Färber wrote: >> Am 27.08.2014 17:42, schrieb Eduardo Habkost: >>> On Wed, Aug 27, 2014 at 04:33:54PM +0200, Paolo Bonzini wrote: >>>> Il 27/08/2014 16:05, Eduardo Habkost ha scritto: >>>>> On Wed, Aug 27, 2014 at 03:36:51PM +0200, Paolo Bonzini wrote: >>>>>> Il 26/08/2014 20:01, Eduardo Habkost ha scritto: >>>>>>> So maybe that's good news, as things can be simpler if we make both TCG >>>>>>> and KVM have similar behavior: >>>>>>> >>>>>>> * qemu64: a conservative default that should work out of the box on >>>>>>> most systems, for both TCG and KVM. That's already the current status, >>>>>>> we just need to document it. >>>>>>> >>>>>>> * -cpu host: for people who want every possible feature to be enabled >>>>>>> (but without cross-version live-migration support). We can easily add >>>>>>> support for "-cpu host" to TCG, too. >>>>>> >>>>>> This means that "-cpu host" has different meanings in KVM and TCG. Is >>>>>> that an advantage or a disadvantage? >>>>> >>>>> It is the same meaning to me: "enable everything that's possible, >>>>> considering what's provided by the underlying accelerator". The "host" >>>>> name is misleading, though, because on KVM it is close to the host CPU, >>>>> but on TCG it depends solely on TCG's capabilities. >>>> >>>> True. It's not very intuitive, but it is the same concept for processor >>>> capabilities. >>>> >>>> Though for some leaves that do not correspond to processor capabilities, >>>> "-cpu host" does set them to the host values. This is not just the >>>> cache model, but also the family/model/stepping/vendor. >>>> >>>> For the TCG case, when running on a Nehalem it would be weird to see a >>>> Nehalem guest with SMAP or ADOX support... I'm not sure it would even >>>> work to have SVM with an Intel vendor. :) >>> >>> In that case, the best family/model/stepping/vendor choice depends on >>> TCG capabilities (defined at compile time), not on the host CPU. >>> >>> ...and that proves your point: if we aren't even using the host CPU >>> family/model/stepping, calling it "-cpu host" doesn't make much sense. >>> If it is so different from the host model, we can call it "qemu64" (and >>> do as you suggests below). >> >> Might that be an opportunity to reconsider a -cpu best or so, >> independent of its implementation, to avoid "host"? > > It depends on what you expect "-cpu best" to mean. I have seen different > meanings being proposed for it. > > IIRC, "best" was proposed to mean "choose the best one from the existing > (predefined) CPU models", not "enable everything possible, not even > looking at the CPU model table". > > Anyway, it makes sense to have a name for the "enable everything" mode > (whatever it is), and simply make "qemu64" an alias to it when in TCG > mode. > > (If we didn't have existing libvirt code assuming "qemu64" is always the > default in QEMU, we could simply get rid of "qemu64" and use better > names. We may get rid of "qemu64" later, but we need to provide a way > for libvirt to stop using it, first.) My "or so" referring to, e.g., -cpu optimum or -cpu maximum or whatever we come up with that is a little more telling than "qemu64" or "host". Andreas -- SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg -- 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
