Re: [PATCH v2 2/7] Documentation: s390-diag.rst: make diag500 a generic KVM hypercall

[David Hildenbrand]

On 14.10.24 20:04, Heiko Carstens wrote:
> On Mon, Oct 14, 2024 at 04:46:14PM +0200, David Hildenbrand wrote:
> > Let's make it a generic KVM hypercall, allowing other subfunctions to
> > be more independent of virtio.
> >
> > This is a preparation for documenting a new hypercall.
> >
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   Documentation/virt/kvm/s390/s390-diag.rst | 15 ++++++++-------
> >   1 file changed, 8 insertions(+), 7 deletions(-)
>
> ...
>
> > -DIAGNOSE function code 'X'500' - KVM virtio functions
> > ------------------------------------------------------
> > +DIAGNOSE function code 'X'500' - KVM functions
> > +----------------------------------------------
> >   
> > -If the function code specifies 0x500, various virtio-related functions
> > -are performed.
> > +If the function code specifies 0x500, various KVM-specific functions
> > +are performed, including virtio functions.
> >   
> > -General register 1 contains the virtio subfunction code. Supported
> > -virtio subfunctions depend on KVM's userspace. Generally, userspace
> > -provides either s390-virtio (subcodes 0-2) or virtio-ccw (subcode 3).
> > +General register 1 contains the subfunction code. Supported subfunctions
> > +depend on KVM's userspace. Regarding virtio subfunctions, generally
> > +userspace provides either s390-virtio (subcodes 0-2) or virtio-ccw
> > +(subcode 3).
>
> Reading this file leaves a number of questions open: how does one know
> which subcodes are supported, and what happens if an unsupported
> subcode is used?

Currently they have to be sensed
> I'm afraid there is no indication available and the only way to figure
> out is to try and if it is unsupported the result is a specification
> exception. Is that correct?

Yes exactly.

> If so, it would be nice to document that too; but that is not
> necessarily your problem.

I can squash:

diff --git a/Documentation/virt/kvm/s390/s390-diag.rst b/Documentation/virt/kvm/s390/s390-diag.rst
index d9b7c6cbc99e..48a326d41cc0 100644
--- a/Documentation/virt/kvm/s390/s390-diag.rst
+++ b/Documentation/virt/kvm/s390/s390-diag.rst
@@ -50,6 +50,9 @@ Upon completion of the DIAGNOSE instruction, general register 2 contains
 the function's return code, which is either a return code or a subcode
 specific value.
 
+If the specified subfunction is not supported, a SPECIFICATION exception
+will be triggered.
+
 Subcode 0 - s390-virtio notification and early console printk
     Handled by userspace.
 


> I guess we won't see too many new diag 500 subcodes, or would it make
> sense to implement some query subcode?

In the context of STORAGE LIMIT, a "query" subfunction is not really beneficial:

it's either one invocation of "query", conditionally followed by one invocation of "STORAGE LIMIT"
vs. one invocation of "STORAGE LIMIT".

Once there might be a bunch of other subfunctions, a "query" might make more sense.

Thanks!

--
Cheers,

David / dhildenb