Re: [PATCH] pseries: fix kvmppc_set_fwnmi()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 7/24/20 10:35 AM, Laurent Vivier wrote:
> QEMU issues the ioctl(KVM_CAP_PPC_FWNMI) on the first vCPU.
> 
> If the first vCPU is currently running, the vCPU mutex is held
> and the ioctl() cannot be done and waits until the mutex is released.
> This never happens and the VM is stuck.
> 
> To avoid this deadlock, issue the ioctl on the same vCPU doing the
> RTAS call.
> 
> The problem can be reproduced by booting a guest with several vCPUs
> (the probability to have the problem is (n - 1) / n,  n = # of CPUs),
> and then by triggering a kernel crash with "echo c >/proc/sysrq-trigger".
> 
> On the reboot, the kernel hangs after:
> 
> ...
> [    0.000000] -----------------------------------------------------
> [    0.000000] ppc64_pft_size    = 0x0
> [    0.000000] phys_mem_size     = 0x48000000
> [    0.000000] dcache_bsize      = 0x80
> [    0.000000] icache_bsize      = 0x80
> [    0.000000] cpu_features      = 0x0001c06f8f4f91a7
> [    0.000000]   possible        = 0x0003fbffcf5fb1a7
> [    0.000000]   always          = 0x00000003800081a1
> [    0.000000] cpu_user_features = 0xdc0065c2 0xaee00000
> [    0.000000] mmu_features      = 0x3c006041
> [    0.000000] firmware_features = 0x00000085455a445f
> [    0.000000] physical_start    = 0x8000000
> [    0.000000] -----------------------------------------------------
> [    0.000000] numa:   NODE_DATA [mem 0x47f33c80-0x47f3ffff]
> 
> Fixes: ec010c00665b ("ppc/spapr: KVM FWNMI should not be enabled until guest requests it")
> Cc: npiggin@gmail.com
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> ---
>  hw/ppc/spapr_rtas.c  | 2 +-
>  target/ppc/kvm.c     | 3 +--
>  target/ppc/kvm_ppc.h | 4 ++--
>  3 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
> index bcac0d00e7b6..513c7a84351b 100644
> --- a/hw/ppc/spapr_rtas.c
> +++ b/hw/ppc/spapr_rtas.c
> @@ -438,7 +438,7 @@ static void rtas_ibm_nmi_register(PowerPCCPU *cpu,
>      }
>  
>      if (kvm_enabled()) {
> -        if (kvmppc_set_fwnmi() < 0) {
> +        if (kvmppc_set_fwnmi(cpu) < 0) {
>              rtas_st(rets, 0, RTAS_OUT_NOT_SUPPORTED);
>              return;
>          }
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 2692f76130aa..d85ba8ffe00b 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2071,9 +2071,8 @@ bool kvmppc_get_fwnmi(void)
>      return cap_fwnmi;
>  }
>  
> -int kvmppc_set_fwnmi(void)
> +int kvmppc_set_fwnmi(PowerPCCPU *cpu)
>  {
> -    PowerPCCPU *cpu = POWERPC_CPU(first_cpu);
>      CPUState *cs = CPU(cpu);
>  
>      return kvm_vcpu_enable_cap(cs, KVM_CAP_PPC_FWNMI, 0);
> diff --git a/target/ppc/kvm_ppc.h b/target/ppc/kvm_ppc.h
> index 701c0c262be2..72e05f1cd2fc 100644
> --- a/target/ppc/kvm_ppc.h
> +++ b/target/ppc/kvm_ppc.h
> @@ -28,7 +28,7 @@ void kvmppc_set_papr(PowerPCCPU *cpu);
>  int kvmppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr);
>  void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy);
>  bool kvmppc_get_fwnmi(void);
> -int kvmppc_set_fwnmi(void);
> +int kvmppc_set_fwnmi(PowerPCCPU *cpu);
>  int kvmppc_smt_threads(void);
>  void kvmppc_error_append_smt_possible_hint(Error *const *errp);
>  int kvmppc_set_smt_threads(int smt);
> @@ -169,7 +169,7 @@ static inline bool kvmppc_get_fwnmi(void)
>      return false;
>  }
>  
> -static inline int kvmppc_set_fwnmi(void)
> +static inline int kvmppc_set_fwnmi(PowerPCCPU *cpu)
>  {
>      return -1;
>  }
>