Re: [PATCH] KVM: s390: get rid of register asm usage

Christian Borntraeger
2021-06-22T07:57:53Z

On 22.06.21 09:53, Thomas Huth wrote:
> On 22/06/2021 09.50, Christian Borntraeger wrote:
>>
>>
>> On 22.06.21 09:46, Thomas Huth wrote:
>>> On 22/06/2021 09.43, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 22.06.21 09:36, Thomas Huth wrote:
>>>>> On 21/06/2021 16.03, Heiko Carstens wrote:
>>>>>> Using register asm statements has been proven to be very error prone,
>>>>>> especially when using code instrumentation where gcc may add function
>>>>>> calls, which clobbers register contents in an unexpected way.
>>>>>>
>>>>>> Therefore get rid of register asm statements in kvm code, even though
>>>>>> there is currently nothing wrong with them. This way we know for sure
>>>>>> that this bug class won't be introduced here.
>>>>>>
>>>>>> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>>> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
>>>>>> ---
>>>>>>   arch/s390/kvm/kvm-s390.c | 18 +++++++++---------
>>>>>>   1 file changed, 9 insertions(+), 9 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>> index 1296fc10f80c..4b7b24f07790 100644
>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>>> @@ -329,31 +329,31 @@ static void allow_cpu_feat(unsigned long nr)
>>>>>>   static inline int plo_test_bit(unsigned char nr)
>>>>>>   {
>>>>>> -    register unsigned long r0 asm("0") = (unsigned long) nr | 0x100;
>>>>>> +    unsigned long function = (unsigned long) nr | 0x100;
>>>>>>       int cc;
>>>>>>       asm volatile(
>>>>>> +        "    lgr    0,%[function]\n"
>>>>>>           /* Parameter registers are ignored for "test bit" */
>>>>>>           "    plo    0,0,0,0(0)\n"
>>>>>>           "    ipm    %0\n"
>>>>>>           "    srl    %0,28\n"
>>>>>>           : "=d" (cc)
>>>>>> -        : "d" (r0)
>>>>>> -        : "cc");
>>>>>> +        : [function] "d" (function)
>>>>>> +        : "cc", "0");
>>>>>>       return cc == 0;
>>>>>>   }
>>>>>>   static __always_inline void __insn32_query(unsigned int opcode, u8 *query)
>>>>>>   {
>>>>>> -    register unsigned long r0 asm("0") = 0;    /* query function */
>>>>>> -    register unsigned long r1 asm("1") = (unsigned long) query;
>>>>>> -
>>>>>>       asm volatile(
>>>>>> -        /* Parameter regs are ignored */
>>>>>> +        "    lghi    0,0\n"
>>>>>> +        "    lgr    1,%[query]\n"
>>>>>> +        /* Parameter registers are ignored */
>>>>>>           "    .insn    rrf,%[opc] << 16,2,4,6,0\n"
>>>>>>           :
>>>>>> -        : "d" (r0), "a" (r1), [opc] "i" (opcode)
>>>>>> -        : "cc", "memory");
>>>>>> +        : [query] "d" ((unsigned long)query), [opc] "i" (opcode)
: "cc", "memory", "0", "1");
>>>>>
>>>>> Wouldn't it be better to keep the "a" constraint instead of "d" to avoid that the compiler ever passes the "query" value in r0 ?
>>>>> Otherwise the query value might get trashed if it is passed in r0...
>>>>
>>>> I first thought the same, but if you look closely the value is only used by the lgr, to load
>>>> the value finally into r1. So d is correct as lgr can take all registers.
>>>
>>> But what about the "lghi    0,0" right in front of it? ... I've got the feeling that I'm missing something here...
>>
>> It does load an immediate value of 0 into register 0. Are you afraid of an early clobber if
>> gcc decides to use r0 for query?
>
> Right, that was my concern. It's a "static __always_inline" function, so can we be sure that query is still always located in a register that is reserved for parameters (i.e. >= r2) ?

Adding Andreas. I know that gcc provides the early clobber "&" if an output is written before all imputs are used.
but in this case it is not an output but a clobbered register.
The clobber list contains 0 and 1, so I guess gcc will never use those as input register.
Andreas, is that correct?

PS: Of course switching both instructions does not hurt