Re: [Xen-devel] [PATCH for-4.12 V3] x86/altp2m: fix HVMOP_altp2m_set_domain_state race

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 2/11/19 12:10 PM, Jan Beulich wrote:
> > > > On 11.02.19 at 10:13, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -2150,13 +2150,13 @@ static bool_t vmx_is_singlestep_supported(void)
> >       return !!cpu_has_monitor_trap_flag;
> >   }
> >   
> > -static void vmx_vcpu_update_eptp(struct vcpu *v)
> > +static void vmx_vcpu_update_eptp(struct vcpu *v, bool altp2m_enabled)
> >   {
> >       struct domain *d = v->domain;
> >       struct p2m_domain *p2m = NULL;
> >       struct ept_data *ept;
> >   
> > -    if ( altp2m_active(d) )
> > +    if ( altp2m_enabled )
> >           p2m = p2m_get_altp2m(v);
> >       if ( !p2m )
> >           p2m = p2m_get_hostp2m(d);
> With the change you now make to p2m_get_altp2m(), this looks to be
> a benign change. Which to me would suggest to either leave the code
> alone, or to drop the if() (but - again - not its body) altogether. At
> which point the code could be further streamlined, as then the NULL
> initializer can go away and the assignment (or then perhaps initializer)
> could become "p2m = p2m_get_altp2m(v) ?: p2m_get_hostp2m(d)".
> (Generally I'd recommend to leave out the change here, and do the
> transformation in a follow-on patch.)
Thanks for noticing, actually this appears to invalidate the whole 
purpose of the patch (I should have tested this more before sumbitting 
V3, sorry).
The whole point of the new boolean is to have p2m assigned an altp2m 
regardless of altp2m_active() (hence the change) - which now no longer 
happens. I got carried away with this change.
The fact that this is so easy to get wrong is the reason why I've 
preferred the domain_pause() solution. There appears to be no clean way 
to fix this otherwise, and if this is so easy to misunderstand it'll 
break just as easily with further changes.
I suppose I could just pass the bool along to p2m_get_altp2m() (and 
indeed remove the if())...

Thanks,
Razvan

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