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

[Christian Borntraeger <borntraeger@de.ibm.com>]

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)
> 
> 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.