Re: [Xen-devel] [XEN PATCH v2] x86/vm_event: add short-circuit for breakpoints (aka, "fast single step")

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.12.2019 06:53, Sergey Kovalev wrote:
> When using DRAKVUF (or another system using altp2m with shadow pages similar
> to what is described in
> https://xenproject.org/2016/04/13/stealthy-monitoring-with-xen-altp2m),
> after a breakpoint is hit the system switches to the default
> unrestricted altp2m view with singlestep enabled. When the singlestep
> traps to Xen another vm_event is sent to the monitor agent, which then
> normally disables singlestepping and switches the altp2m view back to
> the restricted view.
> 
> This patch short-circuiting that last part so that it doesn't need to send the
> vm_event out for the singlestep event and should switch back to the restricted
> view in Xen automatically.
> 
> This optimization gains about 35% speed-up.
> 
> Was tested on Debian branch of Xen 4.12. See at:
> https://github.com/skvl/xen/tree/debian/knorrie/4.12/fast-singlestep
> 
> Rebased on master:
> https://github.com/skvl/xen/tree/fast-singlestep
> 
> Signed-off-by: Sergey Kovalev <valor@xxxxxxx>
> ---
>   xen/arch/x86/hvm/hvm.c         | 12 ++++++++++++
>   xen/arch/x86/hvm/monitor.c     |  9 +++++++++
>   xen/arch/x86/vm_event.c        |  8 ++++++--
>   xen/include/asm-x86/hvm/hvm.h  |  1 +
>   xen/include/asm-x86/hvm/vcpu.h |  4 ++++
>   xen/include/public/vm_event.h  | 10 ++++++++++
>   6 files changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 47573f71b8..4999569503 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -5126,6 +5126,18 @@ void hvm_toggle_singlestep(struct vcpu *v)
>       v->arch.hvm.single_step = !v->arch.hvm.single_step;
>   }
> 
> +void hvm_fast_singlestep(struct vcpu *v, uint16_t p2midx)
> +{
> +    ASSERT(atomic_read(&v->pause_count));
> +
> +    if ( !hvm_is_singlestep_supported() )
> +        return;
> +
> +    v->arch.hvm.single_step = true;
> +    v->arch.hvm.fast_single_step.enabled = true;
> +    v->arch.hvm.fast_single_step.p2midx = p2midx;

Perhaps better at least range check p2midx before storing?

> --- a/xen/arch/x86/vm_event.c
> +++ b/xen/arch/x86/vm_event.c
> @@ -61,7 +61,8 @@ void vm_event_cleanup_domain(struct domain *d)
>   void vm_event_toggle_singlestep(struct domain *d, struct vcpu *v,
>                                   vm_event_response_t *rsp)
>   {
> -    if ( !(rsp->flags & VM_EVENT_FLAG_TOGGLE_SINGLESTEP) )
> +    if ( !(rsp->flags & VM_EVENT_FLAG_TOGGLE_SINGLESTEP ||
> +           rsp->flags & VM_EVENT_FLAG_FAST_SINGLESTEP) )

This wants parentheses added, or re-writing as

    if ( !(rsp->flags & (VM_EVENT_FLAG_TOGGLE_SINGLESTEP |
                         VM_EVENT_FLAG_FAST_SINGLESTEP)) )

Also your patch has come through mangled, reminding me of a problem
I had with Thunderbird after initially having switched to it. There
are line length / wrapping settings you may need to play with to
avoid it inserting extra blanks (I'm sorry, I don't really recall
which one(s) it was.).

Jan

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