Re: [Qemu-devel] [PATCH v5] i386: Introduce ARAT CPU feature

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

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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.
Live-migration or VM pausing are external effects on all timers of the
guest, not only the APIC. However, none of them cause a wakeup miss -
provided the host decides to resume the guest eventually.

Jan


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