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

["Radim Krčmář" <rkrcmar@redhat.com>]

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 <rkrcmar@redhat.com>
> 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).

commit b6ac069532218027f2991cba01d7a72a200688b0
Author: Radim Krčmář <rkrcmar@redhat.com>
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: stable@vger.kernel.org
    Fixes: f30ebc312ca9 ("KVM: x86: optimize some accesses to LVTT and SPIV")
    Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
    Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>