On Wed, May 09, 2012 at 10:42:26AM +0200, Alexander Graf wrote: > > > On 09.05.2012, at 10:14, Gleb Natapov <gleb@xxxxxxxxxx> wrote: > > > 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? > > You have to enable the in-kernel apic to use it, at which point the kernel knows it's in use, right? > > > > >>> - 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 > > Doesn't sound like the greatest design - unless you deprecate the non-in-kernel apic case. > You mean terminating MSRs in kernel does not sound like the greatest design? I do not disagree. That is why IMO kernel can't filter out TSC-deadline and x2apic like you suggest. > > wouldn't make it into ABI. > > > > > > -- > > Gleb. -- 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
