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

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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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> 
> 
> [...]
> 
> 
> > +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.
> 
> 
> [...]






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