Re: [Xen-devel] [PATCH v3 2/2] vmx/hap: optimize CR4 trapping

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

On Fri, Feb 23, 2018 at 04:56:38AM +0000, Tian, Kevin wrote:
> > From: Roger Pau Monne [mailto:roger.pau@xxxxxxxxxx]
> > Sent: Tuesday, February 20, 2018 4:57 PM
> > 
> > There a bunch of bits in CR4 that should be allowed to be set directly
> > by the guest without requiring Xen intervention, currently this is
> > already done by passing through guest writes into the CR4 used when
> > running in non-root mode, but taking an expensive vmexit in order to
> > do so.
> > 
> > xenalyze reports the following when running a PV guest in shim mode:
> > 
> >  CR_ACCESS             3885950  6.41s 17.04%  3957 cyc { 2361| 3378| 7920}
> >    cr4  3885940  6.41s 17.04%  3957 cyc { 2361| 3378| 7920}
> >    cr3        1  0.00s  0.00%  3480 cyc { 3480| 3480| 3480}
> >      *[  0]        1  0.00s  0.00%  3480 cyc { 3480| 3480| 3480}
> >    cr0        7  0.00s  0.00%  7112 cyc { 3248| 5960|17480}
> >    clts        2  0.00s  0.00%  4588 cyc { 3456| 5720| 5720}
> > 
> > After this change this turns into:
> > 
> >  CR_ACCESS                  12  0.00s  0.00%  9972 cyc { 3680|11024|24032}
> >    cr4        2  0.00s  0.00% 17528 cyc {11024|24032|24032}
> >    cr3        1  0.00s  0.00%  3680 cyc { 3680| 3680| 3680}
> >      *[  0]        1  0.00s  0.00%  3680 cyc { 3680| 3680| 3680}
> >    cr0        7  0.00s  0.00%  9209 cyc { 4184| 7848|17488}
> >    clts        2  0.00s  0.00%  8232 cyc { 5352|11112|11112}
> > 
> > Note that this optimized trapping is currently only applied to guests
> > running with HAP on Intel hardware. If using shadow paging more CR4
> > bits need to be unconditionally trapped, which makes this approach
> > unlikely to yield any important performance improvements.
> > 
> > Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > Acked-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> > ---
> > Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> > Cc: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> > ---
> > Changes since v2:
> >  - Use cr4_host_mask.
> > 
> > Changes since v1:
> >  - Use the mask_cr variable in order to cache the cr4 mask.
> >  - Take into account write_ctrlreg_mask when introspection is enabled.
> > ---
> >  xen/arch/x86/hvm/vmx/vmx.c  | 39
> > +++++++++++++++++++++++++++++++++++++++
> >  xen/arch/x86/hvm/vmx/vvmx.c |  2 ++
> >  xen/arch/x86/monitor.c      |  5 +++--
> >  3 files changed, 44 insertions(+), 2 deletions(-)
> > 
> > diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> > index 5cd689e823..27cbbe8823 100644
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -1684,6 +1684,36 @@ static void vmx_update_guest_cr(struct vcpu *v,
> > unsigned int cr)
> >          }
> > 
> >          __vmwrite(GUEST_CR4, v->arch.hvm_vcpu.hw_cr[4]);
> > +
> > +        if ( !paging_mode_hap(v->domain) )
> > +            /*
> > +             * Shadow path has not been optimized because it requires
> > +             * unconditionally trapping more CR4 bits, at which point the
> > +             * performance benefit of doing this is quite dubious.
> > +             */
> > +            v->arch.hvm_vcpu.cr4_host_mask = ~0UL;
> > +        else
> > +        {
> > +            /*
> > +             * Update CR4 host mask to only trap when the guest tries to 
> > set
> > +             * bits that are controlled by the hypervisor.
> > +             */
> > +            v->arch.hvm_vcpu.cr4_host_mask = HVM_CR4_HOST_MASK |
> > X86_CR4_PKE |
> > +                                             ~hvm_cr4_guest_valid_bits(v, 
> > 0);
> > +            v->arch.hvm_vcpu.cr4_host_mask |= v-
> > >arch.hvm_vmx.vmx_realmode ?
> > +                                              X86_CR4_VME : 0;
> > +            v->arch.hvm_vcpu.cr4_host_mask |= !hvm_paging_enabled(v) ?
> > +                                              (X86_CR4_PSE | X86_CR4_SMEP |
> > +                                               X86_CR4_SMAP)
> > +                                              : 0;
> > +            if ( v->domain->arch.monitor.write_ctrlreg_enabled &
> > +                 monitor_ctrlreg_bitmask(VM_EVENT_X86_CR4) )
> > +                v->arch.hvm_vcpu.cr4_host_mask |=
> > +                ~v->domain-
> > >arch.monitor.write_ctrlreg_mask[VM_EVENT_X86_CR4];
> > +
> > +        }
> > +        __vmwrite(CR4_GUEST_HOST_MASK, v-
> > >arch.hvm_vcpu.cr4_host_mask);
> > +
> 
> what about doing a comparison to avoid vmwrite if nothing changed?

I can do that, but it doesn't seem like other writes in the same
function do this (see GUEST_CR4 unconditional write above for
example).

Is it really better from a performance PoV to avoid the write? (or
take the penalty of doing a read + write?)

Thanks, Roger.

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