> -----Original Message----- > From: Xen-devel <xen-devel-boun...@lists.xenproject.org> On Behalf Of Andrew > Cooper > Sent: 03 March 2020 18:25 > To: Xen-devel <xen-devel@lists.xenproject.org> > Cc: Wei Liu <w...@xen.org>; Andrew Cooper <andrew.coop...@citrix.com>; Jan > Beulich <jbeul...@suse.com>; > Anthony PERARD <anthony.per...@citrix.com>; Ian Jackson > <ian.jack...@citrix.com>; Roger Pau Monné > <roger....@citrix.com> > 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 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
