On Wed, May 09, 2012 at 12:07:04AM +0200, Alexander Graf wrote: > > On 08.05.2012, at 22:14, Eduardo Habkost wrote: > > > On Tue, May 08, 2012 at 02:58:11AM +0200, Alexander Graf wrote: > >> On 07.05.2012, at 20:21, Eduardo Habkost wrote: > >> > >>> > >>> Andre? Are you able to help to answer the question below? > >>> > >>> I would like to clarify what's the expected behavior of "-cpu host" to > >>> be able to continue working on it. I believe the code will need to be > >>> fixed on either case, but first we need to figure out what are the > >>> expectations/requirements, to know _which_ changes will be needed. > >>> > >>> > >>> On Tue, Apr 24, 2012 at 02:19:25PM -0300, Eduardo Habkost wrote: > >>>> (CCing Andre Przywara, in case he can help to clarify what's the > >>>> expected meaning of "-cpu host") > >>>> > >>> [...] > >>>> I am not sure I understand what you are proposing. Let me explain the > >>>> use case I am thinking about: > >>>> > >>>> - Feature FOO is of type (A) (e.g. just a new instruction set that > >>>> doesn't require additional userspace support) > >>>> - User has a Qemu vesion that doesn't know anything about feature FOO > >>>> - User gets a new CPU that supports feature FOO > >>>> - User gets a new kernel that supports feature FOO (i.e. has FOO in > >>>> GET_SUPPORTED_CPUID) > >>>> - User does _not_ upgrade Qemu. > >>>> - User expects to get feature FOO enabled if using "-cpu host", without > >>>> upgrading Qemu. > >>>> > >>>> The problem here is: to support the above use-case, userspace need a > >>>> probing mechanism that can differentiate _new_ (previously unknown) > >>>> features that are in group (A) (safe to blindly enable) from features > >>>> that are in group (B) (that can't be enabled without an userspace > >>>> upgrade). > >>>> > >>>> In short, it becomes a problem if we consider the following case: > >>>> > >>>> - Feature BAR is of type (B) (it can't be enabled without extra > >>>> userspace support) > >>>> - User has a Qemu version that doesn't know anything about feature BAR > >>>> - User gets a new CPU that supports feature BAR > >>>> - User gets a new kernel that supports feature BAR (i.e. has BAR in > >>>> GET_SUPPORTED_CPUID) > >>>> - User does _not_ upgrade Qemu. > >>>> - User simply shouldn't get feature BAR enabled, even if using "-cpu > >>>> host", otherwise Qemu would break. > >>>> > >>>> If userspace always limited itself to features it knows about, it would > >>>> be really easy to implement the feature without any new probing > >>>> mechanism from the kernel. But that's not how I think users expect "-cpu > >>>> host" to work. Maybe I am wrong, I don't know. I am CCing Andre, who > >>>> introduced the "-cpu host" feature, in case he can explain what's the > >>>> expected semantics on the cases above. > >> > >> Can you think of any feature that'd go into category B? > > > > - TSC-deadline: can't be enabled unless userspace takes care to enable > > the in-kernel irqchip. > > The kernel can check if in-kernel irqchip has it enabled and otherwise mask it out, no? > How kernel should know that userspace does not emulate it? > > - x2apic: ditto. > > Same here. For user space irqchip the kernel side doesn't care. If in-kernel APIC is enabled, check for its capabilities. > Same here. Well, technically both of those features can't be implemented in userspace right now since MSRs are terminated in the kernel, but I wouldn't make it into ABI. -- Gleb. -- 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
