Re: [PATCH v2 01/10] KVM: x86/mmu: Change tdp_mmu to a read-only parameter

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 20, 2022, David Matlack wrote:
> On Thu, Sep 1, 2022 at 9:47 AM David Matlack <dmatlack@google.com> wrote:
> > On Tue, Aug 30, 2022 at 3:12 AM Huang, Kai <kai.huang@intel.com> wrote:
> > > On Fri, 2022-08-26 at 16:12 -0700, David Matlack wrote:
> [...]
> > > > +#else
> > > > +/* TDP MMU is not supported on 32-bit KVM. */
> > > > +const bool tdp_mmu_enabled;
> > > > +#endif
> > > > +
> > >
> > > I am not sure by using 'const bool' the compile will always omit the function
> > > call?  I did some experiment on my 64-bit system and it seems if we don't use
> > > any -O option then the generated code still does function call.
> > >
> > > How about just (if it works):
> > >
> > >         #define tdp_mmu_enabled false
> >
> > I can give it a try. By the way, I wonder if the existing code
> > compiles without -O. The existing code relies on a static inline
> > function returning false on 32-bit KVM, which doesn't seem like it
> > would be any easier for the compiler to optimize out than a const
> > bool. But who knows.
> 
> Actually, how did you compile without -O and is that a supported use-case?

Eh, IMO whether or not an unoptimized build is supported is moot.  KVM already
uses "#define <param> 0/false", e.g. see enable_sgx, I don't see any reason to
employ a different method.