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Andrew Cooper
6c5fb129e8
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 nominally 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 in Xen, and make ITSC part of the max CPUID 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 retains the current side effect of enabling ITSC if the guest is marked as nomigrate. 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. Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by: Jan Beulich <jbeulich@suse.com> Acked-by: Ian Jackson <Ian.Jackson@citrix.com>