Re: [Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

[Ian Campbell]

On Mon, 2015-09-21 at 16:31 +0300, Razvan Cojocaru wrote:
> diff --git a/tools/libxc/include/xenctrl.h
> b/tools/libxc/include/xenctrl.h
> index 3482544..3bfa00b 100644
> --- a/tools/libxc/include/xenctrl.h
> +++ b/tools/libxc/include/xenctrl.h
> @@ -2428,6 +2428,18 @@ int xc_monitor_software_breakpoint(xc_interface
> *xch, domid_t domain_id,
>  int xc_monitor_guest_request(xc_interface *xch, domid_t domain_id,
>   bool enable, bool sync);
>  
> +/**
> + * This function enables / disables emulation for each REP for a
> + * REP-compatible instruction.
> + *
> + * @parm xch a handle to an open hypervisor interface.
> + * @parm domain_id the domain id one wants to get the node affinity of.
> + * @parm enable if 0 optimize when possible, else emulate each REP.
> + * @return 0 on success, -1 on failure.
> + */
> +int xc_monitor_emulate_each_rep(xc_interface *xch, domid_t domain_id,
> +bool enable);
> +
>  /***
>   * Memory sharing operations.
>   *
> diff --git a/tools/libxc/xc_monitor.c b/tools/libxc/xc_monitor.c
> index 065669c..b1705dd 100644
> --- a/tools/libxc/xc_monitor.c
> +++ b/tools/libxc/xc_monitor.c
> @@ -143,3 +143,16 @@ int xc_monitor_guest_request(xc_interface *xch,
> domid_t domain_id, bool enable,
>  
>  return do_domctl(xch, );
>  }
> +
> +int xc_monitor_emulate_each_rep(xc_interface *xch, domid_t domain_id,
> +bool enable)
> +{
> +DECLARE_DOMCTL;
> +
> +domctl.cmd = XEN_DOMCTL_monitor_op;
> +domctl.domain = domain_id;
> +domctl.u.monitor_op.op = XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP;
> +domctl.u.monitor_op.event = enable;
> +
> +return do_domctl(xch, );
> +}

This is a plausible binding of a hypercall interface so from the toolside
if the hypervisor people are happy with the nderlying inteface:

Acked-by: Ian Campbell 


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Re: [Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

[Razvan Cojocaru]

On 09/22/2015 06:39 PM, Jan Beulich wrote:
 On 22.09.15 at 17:28,  wrote:
>> On 09/22/2015 06:17 PM, Jan Beulich wrote:
>> On 21.09.15 at 15:31,  wrote:
 --- a/xen/arch/x86/hvm/emulate.c
 +++ b/xen/arch/x86/hvm/emulate.c
 @@ -514,7 +514,8 @@ static int hvmemul_virtual_to_linear(
   * being triggered for repeated writes to a whole page.
   */
  *reps = min_t(unsigned long, *reps,
 -  
 unlikely(current->domain->arch.mem_access_emulate_enabled)
 +  
 unlikely(current->domain->arch.mem_access_emulate_enabled &&
 +   
 current->domain->arch.mem_access_emulate_each_rep)
 ? 1 : 4096);
>>>
>>> unlikely() should not wrap compound conditions, or else its effect of
>>> eliminating mis-predicted branches from the fast path won't be
>>> achieved. In the case here I wonder though whether you couldn't
>>> simply test only ->arch.mem_access_emulate_each_rep.
>>
>> I'll unfold the unlikely().
>>
>> Testing only ->arch.mem_access_emulate_each_rep is what I had done in
>> the original patch, however on Andrew Cooper's suggestion I've now gated
>> this on ->domain->arch.mem_access_emulate_enabled as well.
>>
>> Otherwise, somebody might set mem_access_emulate_each_rep via its
>> xc_monitor_*() call, but then after calling xc_monitor_disable() it
>> would still be in effect, even if the guest is no longer being monitored.
>>
>> If this is not a problem, I'm happy to check just
>> ->arch.mem_access_emulate_each_rep.
> 
> Or perhaps "disabled" should just clear that flag, also to not surprise
> the next one to "enable"?

Fair point, I'll do that.


Thanks,
Razvan

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Re: [Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

[Jan Beulich]

>>> On 21.09.15 at 15:31,  wrote:
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -514,7 +514,8 @@ static int hvmemul_virtual_to_linear(
>   * being triggered for repeated writes to a whole page.
>   */
>  *reps = min_t(unsigned long, *reps,
> -  unlikely(current->domain->arch.mem_access_emulate_enabled)
> +  unlikely(current->domain->arch.mem_access_emulate_enabled 
> &&
> +   current->domain->arch.mem_access_emulate_each_rep)
> ? 1 : 4096);

unlikely() should not wrap compound conditions, or else its effect of
eliminating mis-predicted branches from the fast path won't be
achieved. In the case here I wonder though whether you couldn't
simply test only ->arch.mem_access_emulate_each_rep.

> --- a/xen/arch/x86/monitor.c
> +++ b/xen/arch/x86/monitor.c
> @@ -79,6 +79,12 @@ int monitor_domctl(struct domain *d, struct 
> xen_domctl_monitor_op *mop)
>  return 0;
>  }
>  
> +if ( mop->op == XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP )
> +{
> +d->arch.mem_access_emulate_each_rep = !!mop->event;
> +return 0;
> +}

Considering that there's another "if(mop->op == ...)" right above
this, these two together should become another switch().

Jan


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Re: [Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

[Razvan Cojocaru]

On 09/22/2015 06:17 PM, Jan Beulich wrote:
 On 21.09.15 at 15:31,  wrote:
>> --- a/xen/arch/x86/hvm/emulate.c
>> +++ b/xen/arch/x86/hvm/emulate.c
>> @@ -514,7 +514,8 @@ static int hvmemul_virtual_to_linear(
>>   * being triggered for repeated writes to a whole page.
>>   */
>>  *reps = min_t(unsigned long, *reps,
>> -  unlikely(current->domain->arch.mem_access_emulate_enabled)
>> +  unlikely(current->domain->arch.mem_access_emulate_enabled 
>> &&
>> +   
>> current->domain->arch.mem_access_emulate_each_rep)
>> ? 1 : 4096);
> 
> unlikely() should not wrap compound conditions, or else its effect of
> eliminating mis-predicted branches from the fast path won't be
> achieved. In the case here I wonder though whether you couldn't
> simply test only ->arch.mem_access_emulate_each_rep.

I'll unfold the unlikely().

Testing only ->arch.mem_access_emulate_each_rep is what I had done in
the original patch, however on Andrew Cooper's suggestion I've now gated
this on ->domain->arch.mem_access_emulate_enabled as well.

Otherwise, somebody might set mem_access_emulate_each_rep via its
xc_monitor_*() call, but then after calling xc_monitor_disable() it
would still be in effect, even if the guest is no longer being monitored.

If this is not a problem, I'm happy to check just
->arch.mem_access_emulate_each_rep.

>> --- a/xen/arch/x86/monitor.c
>> +++ b/xen/arch/x86/monitor.c
>> @@ -79,6 +79,12 @@ int monitor_domctl(struct domain *d, struct 
>> xen_domctl_monitor_op *mop)
>>  return 0;
>>  }
>>  
>> +if ( mop->op == XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP )
>> +{
>> +d->arch.mem_access_emulate_each_rep = !!mop->event;
>> +return 0;
>> +}
> 
> Considering that there's another "if(mop->op == ...)" right above
> this, these two together should become another switch().

Understood.


Thanks,
Razvan

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Re: [Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

[Jan Beulich]

>>> On 22.09.15 at 17:28,  wrote:
> On 09/22/2015 06:17 PM, Jan Beulich wrote:
> On 21.09.15 at 15:31,  wrote:
>>> --- a/xen/arch/x86/hvm/emulate.c
>>> +++ b/xen/arch/x86/hvm/emulate.c
>>> @@ -514,7 +514,8 @@ static int hvmemul_virtual_to_linear(
>>>   * being triggered for repeated writes to a whole page.
>>>   */
>>>  *reps = min_t(unsigned long, *reps,
>>> -  
>>> unlikely(current->domain->arch.mem_access_emulate_enabled)
>>> +  
>>> unlikely(current->domain->arch.mem_access_emulate_enabled &&
>>> +   
>>> current->domain->arch.mem_access_emulate_each_rep)
>>> ? 1 : 4096);
>> 
>> unlikely() should not wrap compound conditions, or else its effect of
>> eliminating mis-predicted branches from the fast path won't be
>> achieved. In the case here I wonder though whether you couldn't
>> simply test only ->arch.mem_access_emulate_each_rep.
> 
> I'll unfold the unlikely().
> 
> Testing only ->arch.mem_access_emulate_each_rep is what I had done in
> the original patch, however on Andrew Cooper's suggestion I've now gated
> this on ->domain->arch.mem_access_emulate_enabled as well.
> 
> Otherwise, somebody might set mem_access_emulate_each_rep via its
> xc_monitor_*() call, but then after calling xc_monitor_disable() it
> would still be in effect, even if the guest is no longer being monitored.
> 
> If this is not a problem, I'm happy to check just
> ->arch.mem_access_emulate_each_rep.

Or perhaps "disabled" should just clear that flag, also to not surprise
the next one to "enable"?

Jan


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[Xen-devel] [PATCH V2 1/2] xen, libxc: Fine grained control of REP emulation optimizations

[Razvan Cojocaru]

Previously, if vm_event emulation support was enabled, then REP
optimizations were disabled when emulating REP-compatible
instructions. This patch allows fine-tuning of this behaviour by
providing a dedicated libxc helper function.

Signed-off-by: Razvan Cojocaru 

---
Changes since V1:
 - Renamed the patch, since this is now a xc_monitor_ function,
   so XEN_DOMCTL_emulate_each_rep no longer applies.
 - As suggested by Andrew Cooper, the function is a no-op unless
   mem_access emulation is enabled.
---
 tools/libxc/include/xenctrl.h |   12 
 tools/libxc/xc_monitor.c  |   13 +
 xen/arch/x86/hvm/emulate.c|3 ++-
 xen/arch/x86/monitor.c|6 ++
 xen/include/asm-x86/domain.h  |1 +
 xen/include/public/domctl.h   |1 +
 6 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
index 3482544..3bfa00b 100644
--- a/tools/libxc/include/xenctrl.h
+++ b/tools/libxc/include/xenctrl.h
@@ -2428,6 +2428,18 @@ int xc_monitor_software_breakpoint(xc_interface *xch, 
domid_t domain_id,
 int xc_monitor_guest_request(xc_interface *xch, domid_t domain_id,
  bool enable, bool sync);
 
+/**
+ * This function enables / disables emulation for each REP for a
+ * REP-compatible instruction.
+ *
+ * @parm xch a handle to an open hypervisor interface.
+ * @parm domain_id the domain id one wants to get the node affinity of.
+ * @parm enable if 0 optimize when possible, else emulate each REP.
+ * @return 0 on success, -1 on failure.
+ */
+int xc_monitor_emulate_each_rep(xc_interface *xch, domid_t domain_id,
+bool enable);
+
 /***
  * Memory sharing operations.
  *
diff --git a/tools/libxc/xc_monitor.c b/tools/libxc/xc_monitor.c
index 065669c..b1705dd 100644
--- a/tools/libxc/xc_monitor.c
+++ b/tools/libxc/xc_monitor.c
@@ -143,3 +143,16 @@ int xc_monitor_guest_request(xc_interface *xch, domid_t 
domain_id, bool enable,
 
 return do_domctl(xch, );
 }
+
+int xc_monitor_emulate_each_rep(xc_interface *xch, domid_t domain_id,
+bool enable)
+{
+DECLARE_DOMCTL;
+
+domctl.cmd = XEN_DOMCTL_monitor_op;
+domctl.domain = domain_id;
+domctl.u.monitor_op.op = XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP;
+domctl.u.monitor_op.event = enable;
+
+return do_domctl(xch, );
+}
diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index 5934c72..c39a883 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -514,7 +514,8 @@ static int hvmemul_virtual_to_linear(
  * being triggered for repeated writes to a whole page.
  */
 *reps = min_t(unsigned long, *reps,
-  unlikely(current->domain->arch.mem_access_emulate_enabled)
+  unlikely(current->domain->arch.mem_access_emulate_enabled &&
+   current->domain->arch.mem_access_emulate_each_rep)
? 1 : 4096);
 
 reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);
diff --git a/xen/arch/x86/monitor.c b/xen/arch/x86/monitor.c
index 3d52135..3cb7519 100644
--- a/xen/arch/x86/monitor.c
+++ b/xen/arch/x86/monitor.c
@@ -79,6 +79,12 @@ int monitor_domctl(struct domain *d, struct 
xen_domctl_monitor_op *mop)
 return 0;
 }
 
+if ( mop->op == XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP )
+{
+d->arch.mem_access_emulate_each_rep = !!mop->event;
+return 0;
+}
+
 /*
  * Sanity check
  */
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 59cf826..a088110 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -386,6 +386,7 @@ struct arch_domain
 
 /* Mem_access emulation control */
 bool_t mem_access_emulate_enabled;
+bool_t mem_access_emulate_each_rep;
 } __cacheline_aligned;
 
 #define has_arch_pdevs(d)(!list_empty(&(d)->arch.pdev_list))
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 794d4d5..ae241f2 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -1007,6 +1007,7 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_psr_cmt_op_t);
 #define XEN_DOMCTL_MONITOR_OP_ENABLE0
 #define XEN_DOMCTL_MONITOR_OP_DISABLE   1
 #define XEN_DOMCTL_MONITOR_OP_GET_CAPABILITIES  2
+#define XEN_DOMCTL_MONITOR_OP_EMULATE_EACH_REP  3
 
 #define XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG 0
 #define XEN_DOMCTL_MONITOR_EVENT_MOV_TO_MSR1
-- 
1.7.9.5


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