Re: [PATCH v1 2/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

On Wed, 2022-10-05 at 08:32 +0200, Thomas Huth wrote:
> 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@linux.ibm.com>
> > ---
> > 
> > 
> > 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.

Oh, right.
> 
> 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?

Hmmm, the latter seems a bit ugly since there is so much commonality
with the existing memop. 
> 
>   Thomas
>