Re: [Xen-devel] [PATCH] x86/cpuid: Untangle Invariant TSC handling

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----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.

> 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?

  Paul

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