On 30/09/2022 23.07, Janis Schoetterl-Glausch wrote:
User space can use the MEM_OP ioctl to make storage key checked reads and writes to the guest, however, it has no way of performing atomic, key checked, accesses to the guest. Extend the MEM_OP ioctl in order to allow for this, by adding a cmpxchg mode. For now, support this mode for absolute accesses only. This mode can be use, for example, to set the device-state-change indicator and the adapter-local-summary indicator atomically. Signed-off-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> --- The return value of MEM_OP is: 0 on success, < 0 on generic error (e.g. -EFAULT or -ENOMEM), > 0 if an exception occurred while walking the page tables A cmpxchg failing because the old value doesn't match is neither an error nor an exception, so the question is how best to signal that condition. This is not strictly necessary since user space can compare the value of old after the MEM_OP with the value it set. If they're different the cmpxchg failed. It might be a better user interface if there is an easier way to see if the cmpxchg failed. This patch sets the cmpxchg flag bit to 0 on a successful cmpxchg. This way you can compare against a constant instead of the old old value. This has the disadvantage of being a bit weird, other suggestions welcome.
This also breaks the old API of defining the ioctl as _IOW only ... with your change to the flags field, it effectively gets IOWR instead.
Maybe it would be better to put all the new logic into a new struct and only pass a pointer to that struct in kvm_s390_mem_op, so that the ioctl stays IOW ? ... or maybe even introduce a completely new ioctl for this functionality instead?
Thomas
