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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> 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?

Thanks
Kevin
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel