Re: qemu-kvm: Role of flush_icache_range on PPC

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-09-28 16:26, Alexander Graf wrote:
>
> On 28.09.2011, at 16:23, Jan Kiszka wrote:
>
>> Alex,
>>
>> we have this diff in qemu-kvm:
>>
>> diff --git a/exec.c b/exec.c
>> index c1e045d..f188549 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -3950,6 +3955,11 @@ void cpu_physical_memory_rw(target_phys_addr_t addr, uint8_t *buf,
>>                      cpu_physical_memory_set_dirty_flags(
>>                          addr1, (0xff&  ~CODE_DIRTY_FLAG));
>>                  }
>> +		/* qemu doesn't execute guest code directly, but kvm does
>> +		   therefore flush instruction caches */
>> +		if (kvm_enabled())
>> +		    flush_icache_range((unsigned long)ptr,
>> +				       ((unsigned long)ptr)+l);
>>                  qemu_put_ram_ptr(ptr);
>>              }
>>          } else {
>>
>>
>> flush_icache_range() is doing something only on PPC hosts. So do we need
>> this upstream?
>
> This makes sure that when device emulation overwrites code that is already present in the cache of a CPU, it gets flushed from the icache. I'm fairly sure we want that :). But let's ask Ben and David as well.

/me wondered which write scenario precisely needs this. It could only be 
something synchronous /wrt to some VCPU. Which operations could trigger 
such a write? Does PPC inject software breakpoints in form of trap 
operations or so?

Mmm, according to our ancient recordings, the hunk above was once 
introduced for the sake of IA64: 9dc99a2823. I skipped it in my removal 
patch as it has some non-IA64 effect, at least potentially.

Jan

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