Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest)

[Gleb Natapov]

On Wed, May 09, 2012 at 10:42:26AM +0200, Alexander Graf wrote:
> 
> 
> On 09.05.2012, at 10:14, Gleb Natapov <address@hidden> 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.