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

[Alexander Graf <agraf@suse.de>]

On 09.05.2012, at 10:51, Gleb Natapov wrote:

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

I still don't see why it can't.

Imagine we would filter TSC-deadline and x2apic by default in the kernel - they are not known to exist yet.
Now, we implement TSC-deadline in the kernel. We still filter TSC-deadline out in GET_SUPORTED_CPUID in the kernel. But we provide an interface to user space that says "call me to enable TSC-deadline CPUID, but only if you're using the in-kernel apic"
New user space calls that ioctl when it's using the in-kernel apic, it doesn't when it's using the user space apic.
Old user space doesn't call that ioctl.

So at the end all bits in GET_SUPPORTED_CPUID are consistent with what user space is capable of.


Alex