Re: [PATCH v5] i386: Introduce ARAT CPU feature

[Jan Kiszka <jan.kiszka@web.de>]

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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@siemens.com>
>>>>
>>>> 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@siemens.com>
>>> 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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