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

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

[Cédric Le Goater]

The current way the mask is built can overflow with a 64-bit decrementer.
Use MAKE_64BIT_MASK instead.

Fixes: a8dafa525181 ("target/ppc: Implement large decrementer support for TCG")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
---

 This was found with the QEMU Microwatt machine which uses a 64bit
 decrementer. Here is an experimental tree:

   https://github.com/legoater/qemu/tree/microwatt

 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..a86125c50ff9 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 &= MAKE_64BIT_MASK(0, nr_bits);
     negative = !!(value & (1ULL << (nr_bits - 1)));
     if (negative) {
         value |= (0xFFFFFFFFULL << nr_bits);
-- 
2.31.1

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

[Philippe Mathieu-Daudé]

On 9/13/21 6:27 PM, Cédric Le Goater wrote:
> The current way the mask is built can overflow with a 64-bit decrementer.
> Use MAKE_64BIT_MASK instead.
> 
> Fixes: a8dafa525181 ("target/ppc: Implement large decrementer support for TCG")
> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> ---
> 
>  This was found with the QEMU Microwatt machine which uses a 64bit
>  decrementer. Here is an experimental tree:
> 
>    https://github.com/legoater/qemu/tree/microwatt
> 
>  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..a86125c50ff9 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 &= MAKE_64BIT_MASK(0, nr_bits);

What about:

       value = extract64(value, 0, nr_bits);
       if (value != sextract64(value, 0, nr_bits)) { ...

>      negative = !!(value & (1ULL << (nr_bits - 1)));
>      if (negative) {
>          value |= (0xFFFFFFFFULL << nr_bits);
> 

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

[Luis Fernando Fujita Pires]

> >      bool negative;
> >
> >      /* Truncate value to decr_width and sign extend for simplicity */
> > -    value &= ((1ULL << nr_bits) - 1);
> > +    value &= MAKE_64BIT_MASK(0, nr_bits);
> 
> What about:
> 
>        value = extract64(value, 0, nr_bits);
>        if (value != sextract64(value, 0, nr_bits)) { ...

Or:
    value = extract64(value, 0, nr_bits);
    value = ((target_long)value << (64 - nr_bits)) >> (64 - nr_bits);

Also avoiding the problem with an invalid 64-bit shift with:
> >          value |= (0xFFFFFFFFULL << nr_bits);

--
Luis Pires
Instituto de Pesquisas ELDORADO
Aviso Legal - Disclaimer <https://www.eldorado.org.br/disclaimer.html>

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

[Luis Fernando Fujita Pires]

>     value = extract64(value, 0, nr_bits);
>     value = ((target_long)value << (64 - nr_bits)) >> (64 - nr_bits);

Oops, sorry. 64 might not be correct here. It would depend on the target being either 32 or 64.

--
Luis Pires
Instituto de Pesquisas ELDORADO
Aviso Legal - Disclaimer <https://www.eldorado.org.br/disclaimer.html>

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

[Luis Fernando Fujita Pires]

> >     value = extract64(value, 0, nr_bits);
> >     value = ((target_long)value << (64 - nr_bits)) >> (64 - nr_bits);
> 
> Oops, sorry. 64 might not be correct here. It would depend on the target being
> either 32 or 64.

In fact, sextract already does the sign extension, so this should be all that's needed, right?
    value = sextract<32,64>(value, 0, nr_bits);

--
Luis Pires
Instituto de Pesquisas ELDORADO
Aviso Legal - Disclaimer <https://www.eldorado.org.br/disclaimer.html>

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

[Cédric Le Goater]

On 9/13/21 8:05 PM, Luis Fernando Fujita Pires wrote:
>>>     value = extract64(value, 0, nr_bits);
>>>     value = ((target_long)value << (64 - nr_bits)) >> (64 - nr_bits);
>>
>> Oops, sorry. 64 might not be correct here. It would depend on the target being
>> either 32 or 64.
> 
> In fact, sextract already does the sign extension, so this should be all that's needed, right?
>     value = sextract<32,64>(value, 0, nr_bits);

I am fine with any solution ! Please give a try to this machine  :

  https://github.com/legoater/qemu/tree/microwatt

It's the only one with a 64 bit decrementer :) 

(We should come up with a simpler test case)

Thanks,

C.

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

[Peter Maydell]

On Mon, 13 Sept 2021 at 19:09, Luis Fernando Fujita Pires
<luis.pires@eldorado.org.br> wrote:
>
> > >     value = extract64(value, 0, nr_bits);
> > >     value = ((target_long)value << (64 - nr_bits)) >> (64 - nr_bits);
> >
> > Oops, sorry. 64 might not be correct here. It would depend on the target being
> > either 32 or 64.
>
> In fact, sextract already does the sign extension, so this should be all that's needed, right?
>     value = sextract<32,64>(value, 0, nr_bits);

Indeed, sextract64() is the preferred way to do a sign extension.

(The one thing to watch out for is that you mustn't try to
extract a zero-width field; it will assert if you do.
It also asserts if you specify a field whose start,length
would put either end to the left of bit 63 or the right of
bit 0, but that's less likely than the zero-width case.)

-- PMM