On 2015-06-23 04:50, Wanpeng Li wrote: > > > On 6/22/15 1:38 AM, Jan Kiszka wrote: >> On 2015-06-18 22:21, Eduardo Habkost wrote: >>> On Sun, Jun 07, 2015 at 11:15:08AM +0200, Jan Kiszka wrote: >>>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>>> >>>> ARAT signals that the APIC timer does not stop in power saving states. >>>> As our APICs are emulated, it's fine to expose this feature to guests, >>>> at least when asking for KVM host features or with CPU types that >>>> include the flag. The exact model number that introduced the feature is >>>> not known, but reports can be found that it's at least available since >>>> Sandy Bridge. >>>> >>>> Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>> The code looks good now, but: what are the real consequences of >>> enabling/disabling the flag? What exactly guests use it for? >>> >>> Isn't this going to make guests have additional expectations about the >>> APIC timer that may be broken when live-migrating or pausing the VM? >> ARAT only refers to stopping of the timer in certain power states (which >> we do not even emulate IIRC). In that case, the OS is under risk of >> sleeping forever, thus need to look for a different wakeup source. > > HPET will always be the default broadcast event device I think. But it's unused (under Linux) if per-cpu clockevents are unaffected by CLOCK_EVT_FEAT_C3STOP (x86-only "none-feature"), i.e. have ARAT set. And other guests may have other strategies to deal with missing ARAT. Again, the scenario for me was not a regular setup but some Jailhouse boot of Linux where neither a HPET nor a PIT are available as broadcast sources and Linux therefore refuses to switch to hires mode - in contrast to running on real hardware. Jan
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