Re: [PATCH 3/6] KVM: X86: Make tsc_delta calculation a function of guest tsc

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

[-- Attachment #1: Type: text/plain, Size: 1736 bytes --]

On 2011-04-19 08:46, Roedel, Joerg wrote:
> On Mon, Apr 18, 2011 at 08:02:35PM -0400, Zachary Amsden wrote:
>>>
>>> On Sat, Apr 16, 2011 at 06:09:17PM +0200, Jan Kiszka wrote:
>>>    
>>>> On 2011-03-25 09:44, Joerg Roedel wrote:
>>>>      
>>>    
>>>>> +		tsc_delta = !vcpu->arch.last_guest_tsc ? 0 :
>>>>> +			     tsc - vcpu->arch.last_guest_tsc;
>>>>>        
>>>    
>>>> This patch appears to cause troubles to Linux guests on TSC clocksource
>>>> and APIC highres timer. The first boot after qemu start is always fine,
>>>> but after a reboot the guest timer appears to fire incorrectly or even
>>>> not at all.
>>>>
>>>> Was this patch tested with a guest reboot scenario as well? Does it
>>>> account for the TSC being reset to 0 on reboot?
>>>>      
>>> Hmm, probably the last_guest_tsc is not updated correctly in this
>>> scenario. I will have a look tomorrow.
>>>
>>> 	Joerg
>>>
>>>    
>>
>> To avoid this problem, when the TSC is reset, the overshoot protection 
>> where last_guest_tsc is used is specifically disabled:
>>
>>          /* Reset of TSC must disable overshoot protection below */
>>          vcpu->arch.hv_clock.tsc_timestamp = 0;
>>          vcpu->arch.last_tsc_write = data;
>>          vcpu->arch.last_tsc_nsec = ns;
>>
>> You can probably use the same test - last_guest_tsc is only valid if 
>> tsc_timestamp above != 0.
> 
> Yes, this should also work. But the other way we get rid of
> last_host_tsc which is also a good thing :)

That reminds me: I think we still have the bug that KVM overeagerly
declares the host's TSC unstable after resume from S3 because the TSC
appears to go backward when KVM checks. Or should this have been fixed now?

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 259 bytes --]