Re: [Xen-devel] [PATCH v3 10/16] x86/mm: put nested p2m code under CONFIG_HVM

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Sep 06, 2018 at 05:20:53PM +0100, George Dunlap wrote:
> On 09/04/2018 05:15 PM, Wei Liu wrote:
> > These functions are only useful for nested hvm, which isn't enabled
> > when CONFIG_HVM is false.
> > 
> > Enclose relevant code and fields in CONFIG_HVM. Guard np2m_schedule
> > with nestedhvm_enabled.
> > 
> > Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/domain.c        |  6 ++++--
> >  xen/arch/x86/mm/p2m.c        | 18 ++++++++++++++----
> >  xen/include/asm-x86/domain.h |  2 ++
> >  xen/include/asm-x86/p2m.h    |  2 ++
> >  4 files changed, 22 insertions(+), 6 deletions(-)
> > 
> > diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> > index 313ebb3221..7c945a2428 100644
> > --- a/xen/arch/x86/domain.c
> > +++ b/xen/arch/x86/domain.c
> > @@ -1691,7 +1691,8 @@ void context_switch(struct vcpu *prev, struct vcpu 
> > *next)
> >      {
> >          _update_runstate_area(prev);
> >          vpmu_switch_from(prev);
> > -        np2m_schedule(NP2M_SCHEDLE_OUT);
> > +        if ( nestedhvm_enabled(prevd) )
> > +            np2m_schedule(NP2M_SCHEDLE_OUT);
> >      }
> >  
> >      if ( is_hvm_domain(prevd) && !list_empty(&prev->arch.hvm.tm_list) )
> > @@ -1758,7 +1759,8 @@ void context_switch(struct vcpu *prev, struct vcpu 
> > *next)
> >  
> >          /* Must be done with interrupts enabled */
> >          vpmu_switch_to(next);
> > -        np2m_schedule(NP2M_SCHEDLE_IN);
> > +        if ( nestedhvm_enabled(nextd) )
> > +            np2m_schedule(NP2M_SCHEDLE_IN);
> 
> There's already a nestedhvm_enabled() check first thing in
> np2m_schedule().  How does adding this check help the CONFIG_HVM cause?

np2m_schedule will be gone entirely when !HVM.  Add nestedhvm_enabled
check here, which always evaluates to false when !HVM, makes compiler
able to eliminate the call to np2m_schedule when linking.

> 
> And why not #ifdef out this call, as well as the np2m_schedule()
> functions entirely?

The end result is the same. I'm happy to use either.

Wei.

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