ACK: [SRU][N][PATCH 0/1] MultiVM - L2 guest(s) running stress-ng getting stuck at booting after triggering crash (LP: 2077722)

[Tim Whisonant]

On Wed, Jan 22, 2025 at 01:43:08PM +0100, frank.heimes at canonical.com wrote:
> BugLink: https://bugs.launchpad.net/bugs/2077722
> 
> SRU Justification:
> ==================
> 
> [Impact]
> 
>  * L2 guest(s) (nested virtualization) running stress-ng getting stuck
>    at booting after triggering crash.
> 
>  * When for example having two Ubuntu 24.04 guests and running
>    stress-ng (90% load) on both and triggering crash simultaneously,
>    1st guest gets stuck and does not boot up.
> 
>  * In one of the attempts, both the guests got stuck on booting with console hang.
> 
> [Fix]
> 
>  * a373830f96db a373830f96db288a3eb43a8692b6bcd0bd88dfe1
>    "KVM: PPC: Book3S HV: Mask off LPCR_MER for a vCPU before running it to avoid spurious interrupts"
> 
> [Test Plan]
> 
>  * An Ubuntu Server 24.04 LPAR installation, acting as KVM host,
>    on IBM Power 10 hardware (with nested KVM capable FW1060 or never) is needed.
> 
>  * On top two (or more) KVM guests (now nested), again running 24.04,
>    need to be setup.
> 
>  * Run the attached stress-ng.sh script on both KVM guests.
> 
>  * Trigger crash(es) on both KVM guests at the same time:
>    echo c >/proc/sysrq-trigger
> 
>  * At least one KVM guest (sometimes both) are now stuck while rebooting,
>    without the above patch in place.
> 
> [Where problems could occur]
> 
>  * The changes are in arch/powerpc/kvm/book3s_hv.c only,
>    hence are ppc specific and do not affect any other architecture.
> 
>  * The net changes are more or less only two effective code lines;
>    and additional else case and the explicit masking off the 'MER' bit.
> 
>  * Wrong assumptions may have a different impact on KVM gusts (L0),
>    or interfere with any other virtualization level.
> 
>  * But the commit is an upstream accepted fix
>    [for ec0f6639fa88 ("KVM: PPC: Book3S HV nestedv2: Ensure LPCR_MER bit is passed to the L0")]
>    that landed in kernel 6.12 and was also accepted as stable update
>    for kernels v6.8+.
> 
> [Other Info]
> 
>  * This fix/commit discussed here will be part of the planned
>    target kernel for plucky, hence plucky/25.04 is not affected.
> 
>  * The fix/commit is already included in oracular master-next
>    as 08cbc81b9a61 and included starting with kernel Ubuntu-6.11.0-17.17.
>    
>  * With that only noble needs to be fixed (since this nested virtualization
>    scenario is not supported by Ubuntu prior to noble).
> 
>  * Since the fix is upstream marked as stable update,
>    it would usually be picked up by the kernel team automatically.
>  
>  * But to not loose the 24.04.2 window out of sight I was asked
>    to submit this patch separately.
> 
> Gautam Menghani (1):
>   KVM: PPC: Book3S HV: Mask off LPCR_MER for a vCPU before running it to
>     avoid spurious interrupts
> 
>  arch/powerpc/kvm/book3s_hv.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> -- 
> 2.43.0
> 

Acked-by: Tim Whisonant <tim.whisonant at canonical.com>