Re: [kvm-unit-tests PATCH v3 5/7] s390x: Add library functions for exiting from snippet

Nina Schoetterl-Glausch
2024-06-20T17:16:21Z

On Thu, 2024-06-20 at 18:55 +0200, Claudio Imbrenda wrote:
> On Thu, 20 Jun 2024 16:16:58 +0200
> Nina Schoetterl-Glausch <nsg@linux.ibm.com> wrote:
>
> > It is useful to be able to force an exit to the host from the snippet,
> > as well as do so while returning a value.
> > Add this functionality, also add helper functions for the host to check
> > for an exit and get or check the value.
> > Use diag 0x44 and 0x9c for this.
> > Add a guest specific snippet header file and rename snippet.h to reflect
> > that it is host specific.
> >
> > Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
>
>
> [...]
>
>
> > +static inline void diag44(void)
> > +{
> > + asm volatile("diag 0,0,0x44\n");
> > +}
> > +
> > +static inline void diag9c(uint64_t val)
> > +{
> > + asm volatile("diag %[val],0,0x9c\n"
> > + :
> > + : [val] "d"(val)
> > + );
> > +}
> > +
> > #endif
>
> [...]
>
> > +static inline void force_exit(void)
> > +{
> > + diag44();
> > + mb(); /* allow host to modify guest memory */
> > +}
> > +
> > +static inline void force_exit_value(uint64_t val)
> > +{
> > + diag9c(val);
> > + mb(); /* allow host to modify guest memory */
> > +}
>
> why not adding "memory" to the clobbers of the inline asm? (not a big
> deal, I'm just curious if there is a specific reason for an explicit
> mb())

Mostly a matter of taste I guess.
The diag functions are just convenience wrappers, doing nothing but
executing the diag.
force_exit is a protocol between the host and guest that uses the diags
and adds additional semantics on top.
In theory you could have other use cases where the diags are just a timeslice yield.
>
>
> …