Re: [PATCH v4 4/4] target/ppc: Fix 64-bit decrementer

[David Gibson]

On Mon, Sep 20, 2021 at 08:12:03AM +0200, Cédric Le Goater wrote:
> The current way the mask is built can overflow with a 64-bit decrementer.
> Use sextract64() to extract the signed values and remove the logic to
> handle negative values which has become useless.
> 
> Cc: Luis Fernando Fujita Pires <luis.pires@eldorado.org.br>
> Fixes: a8dafa525181 ("target/ppc: Implement large decrementer support for 
> TCG")
> Signed-off-by: Cédric Le Goater <clg@kaod.org>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

LGTM, but not applying because of the comments on 3/4.

> ---
>  hw/ppc/ppc.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
> index b813ef732ec1..f5d012f860af 100644
> --- a/hw/ppc/ppc.c
> +++ b/hw/ppc/ppc.c
> @@ -821,14 +821,12 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, 
> uint64_t *nextp,
>      CPUPPCState *env = &cpu->env;
>      ppc_tb_t *tb_env = env->tb_env;
>      uint64_t now, next;
> -    bool negative;
> +    int64_t signed_value;
> +    int64_t signed_decr;
>  
>      /* Truncate value to decr_width and sign extend for simplicity */
> -    value &= ((1ULL << nr_bits) - 1);
> -    negative = !!(value & (1ULL << (nr_bits - 1)));
> -    if (negative) {
> -        value |= (0xFFFFFFFFULL << nr_bits);
> -    }
> +    signed_value = sextract64(value, 0, nr_bits);
> +    signed_decr = sextract64(decr, 0, nr_bits);
>  
>      trace_ppc_decr_store(nr_bits, decr, value);
>  
> @@ -850,16 +848,16 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, 
> uint64_t *nextp,
>       * On MSB edge based DEC implementations the MSB going from 0 -> 1 
> triggers
>       * an edge interrupt, so raise it here too.
>       */
> -    if ((value < 3) ||
> -        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && negative) ||
> -        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && negative
> -          && !(decr & (1ULL << (nr_bits - 1))))) {
> +    if ((signed_value < 3) ||
> +        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && signed_value < 0) ||
> +        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && signed_value < 0
> +          && signed_decr >= 0)) {
>          (*raise_excp)(cpu);
>          return;
>      }
>  
>      /* On MSB level based systems a 0 for the MSB stops interrupt delivery */
> -    if (!negative && (tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL)) {
> +    if (signed_value >= 0 && (tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL)) {
>          (*lower_excp)(cpu);
>      }
>  

-- 
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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