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

[Alexander Graf]

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.

> wouldn't make it into ABI.
> 
> 
> --
>            Gleb.