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


From: Liu, Jinsong
Subject: Re: [Qemu-devel] [PATCH 2/2] Expose tsc deadline timer cpuid to guest
Date: Thu, 14 Jun 2012 19:02:03 +0000

Eduardo, Jan

I will update tsc deadline timer patch (at qemu-kvm side) recently.
Have you made a final agreement of the issue 'KVM_CAP_TSC_DEADLINE_TIMER' vs. 
'GET_SUPPORTED_CPUID'?

Thanks,
Jinsong


Eduardo Habkost wrote:
> (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).




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