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

[Heiko Carstens <hca@linux.ibm.com>]

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.

Just my 0.02.