Re: [Qemu-devel] [PATCH 2/2] Expose tsc deadline timer cpuid to guest

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(CCing Andre Przywara, in case he can help to clarify what's the
expected meaning of "-cpu host")

On Tue, Apr 24, 2012 at 06:06:55PM +0200, Jan Kiszka wrote:
> On 2012-04-23 22:02, Eduardo Habkost wrote:
> > On Mon, Apr 23, 2012 at 06:31:25PM +0200, Jan Kiszka wrote:
> >> However, that was how I interpreted this GET_SUPPORTED_CPUID. In fact,
> >> it is used as "kernel or hardware does not _prevent_" already. And in
> >> that sense, it's ok to enable even features that are not in
> >> kernel/hardware hands. We should point out this fact in the documentation.
> > 
> > I see GET_SUPPORTED_CPUID as just a "what userspace can enable because
> > the kernel and the hardware support it (= don't prevent it), as long as
> > userspace has the required support" (meaning A+B). It's a bit like
> > KVM_CHECK_EXTENSION, but with the nice feature that that the
> > capabilities map directly to CPUID bits.
> > 
> > So, it's not clear to me: now you are OK with adding TSC_DEADLINE to
> > GET_SUPPORTED_CPUID?
> > 
> > But we still have the issue of "-cpu host" not knowing what can be
> > safely enabled (without userspace feature-specific setup code), or not.
> > Do you have any suggestion for that? Avi, do you have any suggestion?
> 
> First of all, I bet this was already broken with the introduction of
> x2apic. So TSC deadline won't make it worse. I guess we need to address
> this in userspace, first by masking those features out, later by
> actually emulating them.

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.

> 
> > 
> > And I still don't know the answer to:
> > 
> >>> - How to precisely define the groups (A) and (B)?
> >>>   - "requires additional code only if migration is required" qualifies
> >>>     as (B) or (A)?
> > 
> > 
> > Re: documentation, isn't the following paragraph (already present on
> > api.txt) sufficient?
> > 
> > "The entries returned are the host cpuid as returned by the cpuid
> > instruction, with unknown or unsupported features masked out.  Some
> > features (for example, x2apic), may not be present in the host cpu, but
> > are exposed by kvm if it can emulate them efficiently."
> 
> That suggests such features are always emulated - which is not true.
> They are either emulated, or nothing _prevents_ their emulation by user
> space.

Well... it's a bit more complicated than that: the current semantics are
a bit more than "doesn't prevent", as in theory every single feature can
be emulated by userspace, without any help from the kernel. So, if
"doesn't prevent" were the only criteria, the kernel would set every
single feature bit on GET_SUPPORTED_CPUID, making it not very useful.

At least in the case of x2apic, the kernel is using GET_SUPPORTED_CPUID
to expose a _capability_ too: when x2apic is present on
GET_SUPPORTED_CPUID, userspace knows that in addition to "not
preventing" the feature from being enabled, the kernel is now able to
emulate x2apic (if proper setup is made by userspace). A kernel that
can't emulate x2apic (even if userspace was allowed to emulate it
completely in userspace) would never have x2apic enabled on
GET_SUPPORTED_CPUID.

Like I said previously, in the end GET_SUPPORTED_CPUID is just a
capability querying mechanism like KVM_CHECK_EXTENSION (where each
extension have a specific kernel-capability meaning), but with the nice
feature of being automatically mapped to CPUID bits (with no need for
additional KVM_CAP_* => CPUID translation in userspace).

-- 
Eduardo
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