Re: [PATCH qemu] ppc/kvm: Skip writing DPDES back when in run time state

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

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On Mon, Sep 23, 2019 at 06:41:10PM +1000, Alexey Kardashevskiy wrote:
> On POWER8 systems the Directed Privileged Door-bell Exception State
> register (DPDES) stores doorbell pending status, one bit per a thread
> of a core, set by "msgsndp" instruction. The register is shared among
> threads of the same core and KVM on POWER9 emulates it in a similar way
> (POWER9 does not have DPDES).
> 
> DPDES is shared but QEMU assumes all SPRs are per thread so the only safe
> way to write DPDES back to VCPU before running a guest is doing so
> while all threads are pulled out of the guest so DPDES cannot change.
> There is only one situation when this condition is met: incoming migration
> when all threads are stopped. Otherwise any QEMU HMP/QMP command causing
> kvm_arch_put_registers() (for example printing registers or dumping memory)
> can clobber DPDES in a race with other vcpu threads.
> 
> This changes DPDES handling so it is not written to KVM at runtime.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Applied to ppc-for-4.2, thanks.

> ---
>  target/ppc/kvm.c                | 5 +++++
>  target/ppc/translate_init.inc.c | 9 ++++-----
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 8c5b1f25cc95..820724cc7d15 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -993,6 +993,10 @@ int kvm_arch_put_registers(CPUState *cs, int level)
>          }
>  
>          kvm_set_one_reg(cs, KVM_REG_PPC_TB_OFFSET, &env->tb_env->tb_offset);
> +
> +        if (level > KVM_PUT_RUNTIME_STATE) {
> +            kvm_put_one_spr(cs, KVM_REG_PPC_DPDES, SPR_DPDES);
> +        }
>  #endif /* TARGET_PPC64 */
>      }
>  
> @@ -1297,6 +1301,7 @@ int kvm_arch_get_registers(CPUState *cs)
>          }
>  
>          kvm_get_one_reg(cs, KVM_REG_PPC_TB_OFFSET, &env->tb_env->tb_offset);
> +        kvm_get_one_spr(cs, KVM_REG_PPC_DPDES, SPR_DPDES);
>  #endif
>      }
>  
> diff --git a/target/ppc/translate_init.inc.c b/target/ppc/translate_init.inc.c
> index 0fb11c7ac6da..ba726dec4d00 100644
> --- a/target/ppc/translate_init.inc.c
> +++ b/target/ppc/translate_init.inc.c
> @@ -8200,11 +8200,10 @@ static void gen_spr_power8_dpdes(CPUPPCState *env)
>  {
>  #if !defined(CONFIG_USER_ONLY)
>      /* Directed Privileged Door-bell Exception State, used for IPI */
> -    spr_register_kvm_hv(env, SPR_DPDES, "DPDES",
> -                        SPR_NOACCESS, SPR_NOACCESS,
> -                        &spr_read_generic, SPR_NOACCESS,
> -                        &spr_read_generic, &spr_write_generic,
> -                        KVM_REG_PPC_DPDES, 0x00000000);
> +    spr_register(env, SPR_DPDES, "DPDES",
> +                 SPR_NOACCESS, SPR_NOACCESS,
> +                 &spr_read_generic, SPR_NOACCESS,
> +                 0x00000000);
>  #endif
>  }
>  

-- 
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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