Re: [Xen-devel] [PATCH v2 2/2] vmx/hap: optimize CR4 trapping

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 16, 2018 at 02:30:55PM +0200, Razvan Cojocaru wrote:
> On 02/16/2018 02:10 PM, Roger Pau Monne wrote:
> > diff --git a/xen/arch/x86/monitor.c b/xen/arch/x86/monitor.c
> > index f229e69948..4317658c56 100644
> > --- a/xen/arch/x86/monitor.c
> > +++ b/xen/arch/x86/monitor.c
> > @@ -189,10 +189,11 @@ int arch_monitor_domctl_event(struct domain *d,
> >              ad->monitor.write_ctrlreg_enabled &= ~ctrlreg_bitmask;
> >          }
> >  
> > -        if ( VM_EVENT_X86_CR3 == mop->u.mov_to_cr.index )
> > +        if ( VM_EVENT_X86_CR3 == mop->u.mov_to_cr.index ||
> > +             VM_EVENT_X86_CR4 == mop->u.mov_to_cr.index )
> >          {
> >              struct vcpu *v;
> > -            /* Latches new CR3 mask through CR0 code. */
> > +            /* Latches new CR3 or CR4 mask through CR0 code. */
> >              for_each_vcpu ( d, v )
> >                  hvm_update_guest_cr(v, 0);
> >          }
> 
> Did you, by any chance, test this code with xen-access.c (it already has
> a test for CR4 for the PGE stuff)? I'm not convinced the
> hvm_update_guest_cr(v, 0); call suffices to enable CR4 load exits.

hvm_update_guest_cr is just a wrapper to vmx_update_guest_cr when
using vmx, which will unconditionally re-calculate the CR4 mask when
called with cr == 0 or cr == 4.

I have not tested it with xen-access, but it seems quite
straightforward to me. Are you seeing any other path that could
enable CR4 load accesses without calling hvm_update_guest_cr?

Thanks, Roger.

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