Re: [PATCH/RFC] KVM: s390: vsie: Avoid injecting machine check on signal

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

Am 16.03.26 um 12:56 schrieb Heiko Carstens:
> On Mon, Mar 16, 2026 at 11:39:34AM +0100, Christian Borntraeger wrote:
>> The recent XFER_TO_GUEST_WORK change resulted in a situation, where the
>> vsie code would interpret a signal during work as a machine check during
>> SIE as both use the EINTR return code.
>>
>> Fixes: 2bd1337a1295e ("KVM: s390: Use generic VIRT_XFER_TO_GUEST_WORK functions")
>> Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
>> ---
>>   arch/s390/kvm/vsie.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> ...
> 
>> -	if (rc == -EINTR) {
>> +	if (!skipped && rc == -EINTR) {
>>   		kvm_s390_reinject_machine_check(vcpu, &vsie_page->mcck_info);
>>   		return 0;
> 
> As far as I can tell __SF_SIE_REASON is only used for passing -EINTR in case
> of a machine check, otherwise it is unused (== zero)?
> 
> So the KVM_EXIT* codes don't work instead of -EINTR, since those are uapi, and
> there is nothing that would match a machine check anyway. However I would
> still propose to pass some unique positive number back via the stackframe
> instead of some random negative error number which means nothing.
> 
> That is: change the machine check handler to pass e.g.
> 
> #define KVM_S390_SIE_EXIT_MCCK 1
> 
> 		*((long *)(regs->gprs[15] + __SF_SIE_REASON)) = KVM_S390_SIE_EXIT_MCCK;
> 
> Which would make it much more obvious what happens, and which would also avoid
> such strange bugs.

Yes, this was just a minimal "try to fix and get feedback" and thus an RFC. Will have
a look on your proposal.