Re: [PATCH v7 2/4] s390/mm: Batch PTE updates in lazy MMU mode

[Heiko Carstens]

On Thu, Oct 01, 2026 at 10:00:55AM +0200, Alexander Gordeev wrote:
> On Mon, Aug 24, 2026 at 12:40:48PM +0200, Heiko Carstens wrote:
> > > +static __always_inline bool is_lazy_mmu_active(void)
> > > +{
> > > +	if (__is_defined(__DECOMPRESSOR))
> > > +		return false;
> > > +	if (!get_lowcore()->lazy_mmu_count)
> > > +		return false;
> > 
> > I guess there is opportunity to generate better code here using an
> > alternative and using a flag output constraint too.
> 
> It is only CY and LT instructions that make sense for me.
> Here is what I came up with:

...

> As result instead of gcc code:
> 
> 	lghi	%r8,0
> 	lt	%r1,1032(%r8)
> 	je	...
> 
> We get more or less the same:
> 
> 	lhi	%r1,0
> 	cy	%r1,1032
> 	jne	...
> 
> The only benefit is [zero] "d" (0) could sometimes avoid LHI if a
> zeroed register is already around. But also the KASAN coverage is
> not generated.
> 
> Does it make sense to go ahead whith the alternative?

Not really. But why is lazy_mmu_count 32 bit? Do you expect a nesting
level of 2^32? :) If you would reduce that to one byte you could use
tmy or cliy.