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

Thomas Huth
2021-06-22T07:46:24Z

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)
>>
>> 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...
Heiko, maybe you could at least swap the initialization of r0 and r1, then
I'd feel a little bit better...

Thomas