Re: [Qemu-devel] [PATCH] ppc: fix VTB migration

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Nov 21, 2017 at 09:16:43PM +0100, Laurent Vivier wrote:
> Migration of a system under stress (for example, with
> "stress-ng --numa 2") triggers on the destination
> some kernel watchdog messages like:
> 
> NMI watchdog: BUG: soft lockup - CPU#0 stuck for 3489660870s!
> NMI watchdog: BUG: soft lockup - CPU#1 stuck for 3489660884s!
> 
> This problem appears with the changes introduced by
>     42043e4 spapr: clock should count only if vm is running
> 
> I think this commit only triggers the problem.
> 
> Kernel computes the soft lockup duration using the
> Virtual Timebase register (VTB), not using the Timebase
> Register (TBR, the one 42043e4 stops).
> 
> It appears VTB is not migrated, so this patch adds it in
> the list of the SPRs to migrate, and fixes the problem.
> 
> For the migration, I've tested a migration from qemu-2.8.0 and
> pseries-2.8.0 to a patched master (qemu-2.11.0-rc1). The received
> VTB is 0 (as is it not initialized by qemu-2.8.0), but the value
> seems to be ignored by KVM and a non zero VTB is used by the kernel.
> I have no explanation for that, but as the original problem appears
> only with SMP system under stress I suspect some problems in KVM
> (I think because VTB is shared by all threads of a core).
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Applied to ppc-for-2.11, thanks.

> ---
>  target/ppc/translate_init.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/target/ppc/translate_init.c b/target/ppc/translate_init.c
> index b9c49c22f2..4e11e6f489 100644
> --- a/target/ppc/translate_init.c
> +++ b/target/ppc/translate_init.c
> @@ -8081,10 +8081,10 @@ static void gen_spr_power8_ebb(CPUPPCState *env)
>  /* Virtual Time Base */
>  static void gen_spr_vtb(CPUPPCState *env)
>  {
> -    spr_register(env, SPR_VTB, "VTB",
> +    spr_register_kvm(env, SPR_VTB, "VTB",
>                   SPR_NOACCESS, SPR_NOACCESS,
>                   &spr_read_tbl, SPR_NOACCESS,
> -                 0x00000000);
> +                 KVM_REG_PPC_VTB, 0x00000000);
>  }
>  
>  static void gen_spr_power8_fscr(CPUPPCState *env)

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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