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

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

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)
>>>
>>> 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?

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