Re: [PATCH v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

[Thomas Huth]

On 11/01/2023 11.00, Janis Schoetterl-Glausch wrote:
> On Wed, 2023-01-11 at 08:59 +0100, Thomas Huth wrote:
> > On 10/01/2023 21.26, 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>
> > > ---
> > >    include/uapi/linux/kvm.h |   7 +++
> > >    arch/s390/kvm/gaccess.h  |   3 ++
> > >    arch/s390/kvm/gaccess.c  | 102 +++++++++++++++++++++++++++++++++++++++
> > >    arch/s390/kvm/kvm-s390.c |  41 +++++++++++++++-
> > >    4 files changed, 151 insertions(+), 2 deletions(-)
> [...]
>
> > > +/**
> > > + * cmpxchg_guest_abs_with_key() - Perform cmpxchg on guest absolute address.
> > > + * @kvm: Virtual machine instance.
> > > + * @gpa: Absolute guest address of the location to be changed.
> > > + * @len: Operand length of the cmpxchg, required: 1 <= len <= 16. Providing a
> > > + *       non power of two will result in failure.
> > > + * @old_addr: Pointer to old value. If the location at @gpa contains this value, the
> > > + *         exchange will succeed. After calling cmpxchg_guest_abs_with_key() *@old
> > > + *         contains the value at @gpa before the attempt to exchange the value.
> > > + * @new: The value to place at @gpa.
> > > + * @access_key: The access key to use for the guest access.
> > > + *
> > > + * Atomically exchange the value at @gpa by @new, if it contains *@old.
> > > + * Honors storage keys.
> > > + *
> > > + * Return: * 0: successful exchange
> > > + *         * 1: exchange unsuccessful
> > > + *         * a program interruption code indicating the reason cmpxchg could
> > > + *           not be attempted
> >
> > PGM_OPERATION has also the value 1 ... can we be sure that it never happens
> > here?
>
> Currently yes, only program errors are those explicit in the code,
> PGM_ADDRESSING and PGM_PROTECTION.
>
> > ... maybe it would make sense to use KVM_S390_MEMOP_R_NO_XCHG for
> > return value here instead of 1, too, just to be on the safe side?
>
> I didn't like that idea because I consider KVM_S390_MEMOP_R_NO_XCHG to be
> part of the KVM's api surface and cmpxchg_guest_abs_with_key is an internal
> function that shouldn't concern itself with that.
>
> But being unclear on PGM_OPERATION is indeed ugly.
> Maybe I should just replace "a program interruption code ..." with the specific ones?

Yes, that would help to avoid this confusion. With such a change feel free 
to add:
Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>