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

On 18.12.2019 13:55, Jan Beulich wrote:
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
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:

Rebased on master:

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?
What is the valid range?

--- 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)) )

Thank You very much! I didn't notice that...

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.).
Thank You! I used Thunderbird too :) I will re-check it.
Though I have setup line wrap at 300.

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