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Re: [Xen-devel] [PATCH] x86/cpuid: Untangle Invariant TSC handling
- To: "Durrant, Paul" <pdurrant@xxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 4 Mar 2020 17:31:12 +0000
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- Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 04 Mar 2020 17:31:21 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 04/03/2020 09:33, Durrant, Paul wrote:
>> -----Original Message-----
>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Andrew
>> Cooper
>> Sent: 03 March 2020 18:25
>> To: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Wei Liu <wl@xxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Jan
>> Beulich <JBeulich@xxxxxxxx>;
>> Anthony PERARD <anthony.perard@xxxxxxxxxx>; Ian Jackson
>> <Ian.Jackson@xxxxxxxxxx>; Roger Pau Monné
>> <roger.pau@xxxxxxxxxx>
>> Subject: [Xen-devel] [PATCH] x86/cpuid: Untangle Invariant TSC handling
>>
>> ITSC being visible to the guest is currently implicit with the toolstack
>> unconditionally asking for it, and Xen clipping it based on the vTSC and/or
>> XEN_DOMCTL_disable_migrate settings.
>>
>> This is problematic for several reasons.
>>
>> First, the implicit vTSC behaviour manifests as a real bug on migration to a
>> host with a different frequency, with ITSC but without TSC scaling
>> capabilities, whereby the ITSC feature becomes advertised to the guest. ITSC
>> will disappear again if the guest migrates to server with the same frequency
>> as the original, or to one with TSC scaling support.
>>
>> Secondly, disallowing ITSC unless the guest doesn't migrate is conceptually
>> wrong. It is common to have migration pools of identical hardware, at which
>> point the TSC frequency is the same, and more modern hardware has TSC scaling
>> support anyway. In both cases, it is safe to advertise ITSC and migrate the
>> guest.
>>
>> Remove all implicit logic logic in Xen, and make ITSC part of the max CPUID
> One too many 'logic's there.
Oops.
>
>> policies for guests. Plumb an itsc parameter into xc_cpuid_apply_policy()
>> and
>> have libxl__cpuid_legacy() fill in the two cases where it can reasonably
>> expect ITSC to be safe for the guest to see.
>>
>> This is a behaviour change for TSC_MODE_NATIVE, where the ITSC will now
>> reliably not appear, and for the case where the user explicitly requests
>> ITSC,
>> in which case it will appear even if the guest isn't marked as nomigrate.
>>
> Does this mean a guest that would have seen ITSC on 4.13 may now no longer
> see it in 4.14 or is the TSC_MODE_NATIVE case just the one where the feature
> may erroneously appear after migration?
In general, guests don't get to see ITSC at all, even before this
change. This is something which needs working on, but it is only a
tractable problem in a multi-host toolstack.
After this change, the TSC_MODE_NATIVE case will now not see a
metastable ITSC feature depending on the properties of the host it
happens to be on. It will default to consistently 0, unless overridden
by the toolstack.
~Andrew
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