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

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu, 20 Jun 2024 19:16:05 +0200
Nina Schoetterl-Glausch <nsg@linux.ibm.com> wrote:

> 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.

fair enough

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> > 
> > 
> > [...]  
>