[OpenBIOS] [PATCH] ppc: use rfid when running under a CPU from the 970 family.

[Cédric Le Goater]

On 06/20/2016 03:26 PM, Alexander Graf wrote:
> On 06/20/2016 02:59 PM, Cédric Le Goater wrote:
>> On 06/20/2016 11:49 AM, Mark Cave-Ayland wrote:
>>> On 20/06/16 10:33, Alexander Graf wrote:
>>>
>>>> On 06/20/2016 11:32 AM, Cédric Le Goater wrote:
>>>>> On 06/20/2016 09:39 AM, Alexander Graf wrote:
>>>>>> On 20.06.16 09:34, Cédric Le Goater wrote:
>>>>>>> A recent attempt to restrict the use of rfi to 64bit cpus in qemu
>>>>>>> broke 32bit OpenBIOS when run under a 970.
>>>>>>>
>>>>>>> This is adding a dynamic check on the pvr to choose between rfi and
>>>>>>> rfid. Only the 970 family is supported.
>>>>>>>
>>>>>>> Signed-off-by: Cédric Le Goater <clg at kaod.org>
>>>>>>> ---
>>>>>>>
>>>>>>>    Tested on qemu.
>>>>>>>
>>>>>>>    arch/ppc/qemu/start.S |   25 +++++++++++++++++++++++--
>>>>>>>    arch/ppc/start.S      |   25 +++++++++++++++++++++++--
>>>>>>>    2 files changed, 46 insertions(+), 4 deletions(-)
>>>>>>>
>>>>>>> Index: openbios.git/arch/ppc/qemu/start.S
>>>>>>> ===================================================================
>>>>>>> --- openbios.git.orig/arch/ppc/qemu/start.S
>>>>>>> +++ openbios.git/arch/ppc/qemu/start.S
>>>>>>> @@ -27,6 +27,27 @@
>>>>>>>      #ifdef CONFIG_PPC_64BITSUPPORT
>>>>>>>    +/* When running on ppc64, we cannot use rfi anymore. Let's try to
>>>>>>> + * catch which cpu we are running on and act accordingly. This is
>>>>>>> + * for 970s only.
>>>>>> Do you think you could move the check into the cpu init function and use
>>>>>> a global variable / live patching instead? I don't remember if we need
>>>>>> to handle any faults before we reach that path.
>>>>> So we get an ISI really early, when returning from setup_mmu(), which is
>>>>> the first call in _entry.
>>>> Ok, can we add the check there?
>>> Bear in mind that the code for entry/context switching has been reworked
>>> pending commit whilst we switch the OpenBIOS repository over from SVN to
>>> git. The relevant commits are here:
>>> https://www.coreboot.org/pipermail/openbios/2016-May/009395.html.
>>>
>>> So please consider OpenBIOS git master plus the patchset above to be the
>>> latest code.
>> OK. The first version applies fine. I pushed everything here :
>>
>>     https://github.com/legoater/openbios/commits/master
>>
>> I am still looking at what Alex is asking for : a global variable doing
>> live patching for rfi.
> 
> It's not both - it's either or :).

ah :) I need to grow my live patching skills.

> So either you define a global variable that we can just read from to determine 
> whether we do rfi or rfid 

That is what I started doing and that was much bigger then v1.

> or we live patch. Looking at the code, it's probably
> easiest to live patch even. Just call a function from setup_mmu() that checks 
> PVR and if it's 64bit, loops through (0x0 .. 0x1000) in 32bit steps and checks 
> if *p == INST_RFI. If so, replace with INST_RFID and flush the cache.

Indeed, the hunk below seems to do the trick :

	@@ -524,6 +524,17 @@ setup_mmu(unsigned long ramsize)
	         asm volatile("mtsrin %0,%1" :: "r" (sr_base + i), "r" (j));
	     }
	 
	+    if (is_ppc64()) {
	+#define RFI	0x4c000064
	+#define RFID	0x4c000024
	+        unsigned long addr;
	+
	+        for (addr = 0x2000; addr < 0x3000; addr += 4) {
	+            unsigned long *ptr = (unsigned long *) addr;
	+            if (*ptr == RFI) *ptr = RFID;
	+        }
	+    }
	+
	 #endif

and that's permitted ? :) 

flushing the cache does not seem necessary. I suppose because we did not go 
through this range of instructions yet, I am not sure how qemu handle 
the cache instructions. Need to check.

Thanks, I will bake a patch for it. 

C.