Re: [PATCH v2] target/ppc: Fix 64-bit decrementer

[Cédric Le Goater]

On 9/14/21 11:19 AM, Peter Maydell wrote:
> On Tue, 14 Sept 2021 at 09:56, Cédric Le Goater <clg@kaod.org> wrote:
>>
>> The current way the mask is built can overflow with a 64-bit decrementer.
>> Use sextract64() instead.
>>
>> 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>
>> ---
>>
>>  v2: replaced MAKE_64BIT_MASK by sextract64
>>
>>  hw/ppc/ppc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
>> index 7375bf4fa910..4f14464c9220 100644
>> --- a/hw/ppc/ppc.c
>> +++ b/hw/ppc/ppc.c
>> @@ -876,7 +876,7 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, 
>> uint64_t *nextp,
>>      bool negative;
>>
>>      /* Truncate value to decr_width and sign extend for simplicity */
>> -    value &= ((1ULL << nr_bits) - 1);
>> +    value = sextract64(value, 0, nr_bits);
>>      negative = !!(value & (1ULL << (nr_bits - 1)));
>>      if (negative) {
>>          value |= (0xFFFFFFFFULL << nr_bits);
> 
> I think these lines that say "if negative then force all the
> high bits to one" are also no longer required. That is, this
> entire section of code:
>     value &= ((1ULL << nr_bits) - 1);
>     negative = !!(value & (1ULL << (nr_bits - 1)));
>     if (negative) {
>         value |= (0xFFFFFFFFULL << nr_bits);
>     }
> 
> is an open-coded sign-extension, which can all be replaced with
> the single line
>     value = sextract64(value, 0, nr_bits);

'negative' is used for more tests afterwards but you are right. I will respin 
with more changes. 

I am reluctant in changing too much because this is common code for PPC32
and PPC64. But, hey, I will do my best with the images I have.

> (and also: if nr_bits is 64 then the "<< nr_bits"
> is undefined behaviour.)

That's the initial issue raised by the new little PPC FPGA softcore called 
microwatt as it's using a 64bit decrementer.

Thanks,

C. 

> 
> thanks
> -- PMM
>