[PATCH 1/6] cmd: exception: arm64: fix undefined, add faults

[Heinrich Schuchardt]

On 1/9/22 22:31, Andre Przywara wrote:
> On Sun, 9 Jan 2022 20:08:41 +0100
> Heinrich Schuchardt <heinrich.schuchardt at canonical.com> wrote:
> 
>> On 1/9/22 18:30, Andre Przywara wrote:
>>> The arm64 version of the exception command was just defining the
>>> undefined exception, but actually copied the AArch32 instruction.
>>>
>>> Replace that with an encoding that is guaranteed to be and stay
>>> undefined. Also add instructions to trigger unaligned access faults and
>>> a breakpoint.
>>> This brings ARM64 on par with ARM(32) for the exception command.
>>>
>>> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
>>> ---
>>>    cmd/arm/exception64.c | 42 ++++++++++++++++++++++++++++++++++++++----
>>>    1 file changed, 38 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/cmd/arm/exception64.c b/cmd/arm/exception64.c
>>> index d5de50a0803..1a9730e6aec 100644
>>> --- a/cmd/arm/exception64.c
>>> +++ b/cmd/arm/exception64.c
>>> @@ -12,14 +12,46 @@ static int do_undefined(struct cmd_tbl *cmdtp, int flag, int argc,
>>>    			char *const argv[])
>>>    {
>>>    	/*
>>> -	 * 0xe7f...f.	is undefined in ARM mode
>>> -	 * 0xde..	is undefined in Thumb mode
>>> +	 * Instructions starting with the upper 16 bits all 0 are permanently
>>> +	 * undefined. The lower 16 bits can be used for some kind of immediate.
>>> +	 * --- ARMv8 ARM (ARM DDI 0487G.a C6.2.339: "UDF")
>>>    	 */
>>> -	asm volatile (".word 0xe7f7defb\n");
>>> +	asm volatile (".word 0x00001234\n");
>>> +
>>> +	return CMD_RET_FAILURE;
>>> +}
>>> +
>>> +static int do_unaligned(struct cmd_tbl *cmdtp, int flag, int argc,
>>> +			char *const argv[])
>>> +{
>>> +	/*
>>> +	 * The load acquire instruction requires the data source to be
>>> +	 * naturally aligned, and will fault even if strict alignment fault
>>> +	 * checking is disabled.
>>> +	 * --- ARMv8 ARM (ARM DDI 0487G.a B2.5.2: "Alignment of data accesses")
>>
>> According to DI0487G_b_armv8_arm.pdf available at
>> https://developer.arm.com/documentation/ddi0487/latest the generation of
>> an alignment fault for ldar depends on FEAT_LSE2 (Large System
>> Extensions v2) which is mandatory for ARMv8.4. See p. B2-161.
> 
> Well found, but I wonder if that matters for the SoCs running U-Boot.
> It looks like the Apple M1 is the only one so far and will probably
> stay for a while.

Developers are using U-Boot on Apple M1 already.

> But I can of course check ID_AA64MMFR2_EL1.AT before executing the LDAR,
> and will ask around for a better method to provoke unaligned accesses.

It is sufficient if you update the comment for this function. Returning 
CMD_RET_FAILURE as return value if unaligned access is supported is 
fine. Cf. cmd/riscv/exception.c. (On RISC-V OpenSBI emulates unaligned 
access.)

Maybe we should also add a comment in doc/usage/exception.rst.

Best regards

Heinrich

> 
> Cheers,
> Andre
> 
> 
>>
>> Best regards
>>
>> Heinrich
>>
>>> +	 */
>>> +	asm volatile (
>>> +		"mov	x1, sp\n\t"
>>> +		"orr	x1, x1, #3\n\t"
>>> +		"ldar	x0, [x1]\n"
>>> +		::: "x0", "x1" );
>>> +
>>> +	return CMD_RET_FAILURE;
>>> +}
>>> +
>>> +static int do_breakpoint(struct cmd_tbl *cmdtp, int flag, int argc,
>>> +			 char *const argv[])
>>> +{
>>> +	asm volatile ("brk	#123\n");
>>> +
>>>    	return CMD_RET_FAILURE;
>>>    }
>>>    
>>>    static struct cmd_tbl cmd_sub[] = {
>>> +	U_BOOT_CMD_MKENT(breakpoint, CONFIG_SYS_MAXARGS, 1, do_breakpoint,
>>> +			 "", ""),
>>> +	U_BOOT_CMD_MKENT(unaligned, CONFIG_SYS_MAXARGS, 1, do_unaligned,
>>> +			 "", ""),
>>>    	U_BOOT_CMD_MKENT(undefined, CONFIG_SYS_MAXARGS, 1, do_undefined,
>>>    			 "", ""),
>>>    };
>>> @@ -27,7 +59,9 @@ static struct cmd_tbl cmd_sub[] = {
>>>    static char exception_help_text[] =
>>>    	"<ex>\n"
>>>    	"  The following exceptions are available:\n"
>>> -	"  undefined  - undefined instruction\n"
>>> +	"  breakpoint - breakpoint instruction exception\n"
>>> +	"  unaligned  - unaligned LDAR data abort\n"
>>> +	"  undefined  - undefined instruction exception\n"
>>>    	;
>>>    
>>>    #include <exception.h>
>