Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging for 4.15, Part 1

[Ingo Molnar]

* Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:

> On Tue, Oct 24, 2017 at 11:40:40AM +0200, Ingo Molnar wrote:
> > 
> > * Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > 
> > > On Mon, Oct 23, 2017 at 02:40:14PM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
> > > > 
> > > > > > Making a variable that 'looks' like a constant macro dynamic in a rare Kconfig 
> > > > > > scenario is asking for trouble.
> > > > > 
> > > > > We expect boot-time page mode switching to be enabled in kernel of next
> > > > > generation enterprise distros. It shoudn't be that rare.
> > > > 
> > > > My point remains even with not-so-rare Kconfig dependency.
> > > 
> > > I don't follow how introducing new variable that depends on Kconfig option
> > > would help with the situation.
> > 
> > A new, properly named variable or function (max_physmem_bits or 
> > max_physmem_bits()) that is not all uppercase would make it abundantly clear that 
> > it is not a constant but a runtime value.
> 
> Would we need to rename every uppercase macros that would depend on
> max_physmem_bits()? Like MAXMEM.

MAXMEM isn't used in too many places either - what's the total impact of it?

> > > We would end up with inverse situation: people would use MAX_PHYSMEM_BITS
> > > where the new variable need to be used and we will in the same situation.
> > 
> > It should result in sub-optimal resource allocations worst-case, right?
> 
> I don't think it's the worst case.
> 
> For instance, virt_addr_valid() depends indirectly on it:
> 
>   virt_addr_valid()
>     __virt_addr_valid()
>       phys_addr_valid()
>         boot_cpu_data.x86_phys_bits (initialized with MAX_PHYSMEM_BITS)
> 
> virt_addr_valid() is used in things like implementation /dev/kmem.
> 
> To me it's far more risky than occasional build breakage for
> CONFIG_X86_5LEVEL=y.

So why do we have two variables here, one boot_cpu_data.x86_phys_bits and the 
other MAX_PHYSMEM_BITS - both set once during boot?

I'm trying to find a clean solution for this all - hiding a boot time dependency 
into a constant-looking value doesn't feel clean.

Thanks,

	Ingo