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Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose ts
From: |
Alexander Graf |
Subject: |
Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest) |
Date: |
Wed, 9 May 2012 10:42:26 +0200 |
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.
- [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Eduardo Habkost, 2012/05/07
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Alexander Graf, 2012/05/07
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Eduardo Habkost, 2012/05/08
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Alexander Graf, 2012/05/08
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Gleb Natapov, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest),
Alexander Graf <=
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Gleb Natapov, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Alexander Graf, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Gleb Natapov, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Alexander Graf, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Eduardo Habkost, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Alexander Graf, 2012/05/09
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Gleb Natapov, 2012/05/10
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Alexander Graf, 2012/05/10
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Gleb Natapov, 2012/05/10
- Re: [Qemu-devel] Semantics of "-cpu host" (was Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest), Eduardo Habkost, 2012/05/10