Re: [Qemu-devel] Help - `flush_icache_range' MacPPC

[Scott Wood]

On 02/08/2012 05:42 PM, Alexander Graf wrote:
> 
> On 09.02.2012, at 00:29, Scott Wood wrote:
> 
>> On 02/07/2012 04:57 PM, Alexander Graf wrote:
>>> Eh, this patch:
>>>
>>> diff --git a/tcg/ppc/tcg-target.c b/tcg/ppc/tcg-target.c
>>> index f5d9bf3..f9793e6 100644
>>> --- a/tcg/ppc/tcg-target.c
>>> +++ b/tcg/ppc/tcg-target.c
>>> @@ -1260,6 +1260,17 @@ static void tcg_out_brcond2 (TCGContext *s, const 
>>> TCGArg *args,
>>>     tcg_out_bc (s, (BC | BI (7, CR_EQ) | BO_COND_TRUE), args[5]);
>>> }
>>>
>>> +#ifdef __NetBSD__
>>> +static void flush_icache_range(unsigned long begin, unsigned long end)
>>> +{
>>> +    unsigned int i = begin & ~15UL;
>>> +    for (; i < end; i+=16) {
>>> +        asm("icbi 0,%0" : : "r"(i));
>>> +    }
>>> +    asm("isync");
>>> +}
>>> +#endif
>>
>> What about flushing the data cache first, as the other
>> flush_icache_range does?
> 
> I actually wrote this one from scratch. Ahem.
> 
> So why are we flushing the dcache on the other one? Does the icache
> always fill from memory or can it check if something's in the dcache
> and fetch it from there? If it's the former, then a dcache flush is
> crucial, right.

The icache is allowed to fill directly from memory, without snooping
dcache.  Where it actually fetches from and whether it snoops any data
cache is implementation-dependent.  On our implementations, instruction
fetches do not snoop data cache.  The fetch could hit in main memory, or
in a shared cache such as L3/CPC (e500mc) or L2 (e500v2).

>> Why isn't the cache-utils.h version of flush_icache_range being seen?
>> It's only ppc_init_cacheline_sizes() that is OS-conditional -- shouldn't
>> that be what the undefined reference is for?  Is _ARCH_PPC failing to be
>> defined?
> 
> Oh, there is a cache flush function already there! Hah! I completely missed 
> out on that one.
> 
> I would assume it's either because _ARCH_PPC is failing to be picked
> up (include order?) or because the header file is just not included.
> My motivation in digging into ppc NetBSD issues is rather small
> though.

It looks like GCC sets _ARCH_PPC.  Apparently this isn't happening on
this user's toolchain.

-Scott