Re: [Qemu-devel] Help debugging a regression in KVM Module

[Peter Lieven]

> Am 18.08.2015 um 17:25 schrieb Radim Krčmář <address@hidden>:
> 
> 2015-08-18 16:54+0200, Peter Lieven:
>> After some experiments I was able to find out the bad commit that introduced 
>> the regression:
>> 
>> commit f30ebc312ca9def25650b4e1d01cdb425c310dca
>> Author: Radim Krčmář <address@hidden>
>> Date:   Thu Oct 30 15:06:47 2014 +0100
>> 
>> It seems that this optimisation is not working reliabliy after live 
>> migration. I can't reproduce if
>> I take a 3.19 kernel and revert this single commit.
> 
> Hello, this bug has gone unnoticed for a long time so it is fixed only
> since v4.1 (and v3.19.stable was dead at that point).

thanks for the pointer. i noticed the regression some time ago, but never found 
the time to debug. some distros rely on 3.19 e.g. Ubuntu LTS 14.04.2. I will 
try to ping the maintainer.

Peter

> 
> commit b6ac069532218027f2991cba01d7a72a200688b0
> Author: Radim Krčmář <address@hidden>
> Date:   Fri Jun 5 20:57:41 2015 +0200
> 
>    KVM: x86: fix lapic.timer_mode on restore
> 
>    lapic.timer_mode was not properly initialized after migration, which
>    broke few useful things, like login, by making every sleep eternal.
> 
>    Fix this by calling apic_update_lvtt in kvm_apic_post_state_restore.
> 
>    There are other slowpaths that update lvtt, so this patch makes sure
>    something similar doesn't happen again by calling apic_update_lvtt
>    after every modification.
> 
>    Cc: address@hidden
>    Fixes: f30ebc312ca9 ("KVM: x86: optimize some accesses to LVTT and SPIV")
>    Signed-off-by: Radim Krčmář <address@hidden>
>    Signed-off-by: Marcelo Tosatti <address@hidden>