[PATCH] x86/monitor: add option to disable Xen's pagetable walking on events

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

Add option to the monitor interface to disable walking of the guest pagetable
on certain events. This is a performance optimization for tools that never
require that information or prefer to do it themselves. For example LibVMI
maintains a virtual TLB which is faster to lookup then what Xen does here.

Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
---
 xen/arch/x86/hvm/monitor.c    | 3 +++
 xen/common/monitor.c          | 3 +++
 xen/include/asm-arm/monitor.h | 7 +++++++
 xen/include/asm-x86/domain.h  | 1 +
 xen/include/asm-x86/monitor.h | 6 ++++++
 xen/include/public/domctl.h   | 1 +
 6 files changed, 21 insertions(+)

diff --git a/xen/arch/x86/hvm/monitor.c b/xen/arch/x86/hvm/monitor.c
index e4a09964a0..4c8272ab11 100644
--- a/xen/arch/x86/hvm/monitor.c
+++ b/xen/arch/x86/hvm/monitor.c
@@ -124,6 +124,9 @@ static inline unsigned long gfn_of_rip(unsigned long rip)
     struct segment_register sreg;
     uint32_t pfec = PFEC_page_present | PFEC_insn_fetch;
 
+    if ( curr->domain->arch.monitor.disable_ptwalks )
+        return 0;
+
     if ( hvm_get_cpl(curr) == 3 )
         pfec |= PFEC_user_mode;
 
diff --git a/xen/common/monitor.c b/xen/common/monitor.c
index d5c9ff1cbf..3a808ae7e3 100644
--- a/xen/common/monitor.c
+++ b/xen/common/monitor.c
@@ -58,6 +58,9 @@ int monitor_domctl(struct domain *d, struct 
xen_domctl_monitor_op *mop)
         mop->event = arch_monitor_get_capabilities(d);
         return 0;
 
+    case XEN_DOMCTL_MONITOR_OP_DISABLE_PTWALKS:
+        return arch_monitor_disable_ptwalks(d);
+
     default:
         /* The monitor op is probably handled on the arch-side. */
         return arch_monitor_domctl_op(d, mop);
diff --git a/xen/include/asm-arm/monitor.h b/xen/include/asm-arm/monitor.h
index 7567be66bd..1398a3ee26 100644
--- a/xen/include/asm-arm/monitor.h
+++ b/xen/include/asm-arm/monitor.h
@@ -63,6 +63,13 @@ static inline uint32_t arch_monitor_get_capabilities(struct 
domain *d)
     return capabilities;
 }
 
+static inline
+int arch_monitor_disable_ptwalks(struct domain *d)
+{
+    /* Not supported on ARM. */
+    return -EOPNOTSUPP;
+}
+
 int monitor_smc(void);
 
 #endif /* __ASM_ARM_MONITOR_H__ */
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 3900d7b48b..83f210c750 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -430,6 +430,7 @@ struct arch_domain
          */
         unsigned int inguest_pagefault_disabled                            : 1;
         unsigned int control_register_values                               : 1;
+        unsigned int disable_ptwalks                                       : 1;
         struct monitor_msr_bitmap *msr_bitmap;
         uint64_t write_ctrlreg_mask[4];
     } monitor;
diff --git a/xen/include/asm-x86/monitor.h b/xen/include/asm-x86/monitor.h
index 01c6d63bb9..d9e53a0499 100644
--- a/xen/include/asm-x86/monitor.h
+++ b/xen/include/asm-x86/monitor.h
@@ -100,6 +100,12 @@ static inline uint32_t 
arch_monitor_get_capabilities(struct domain *d)
     return capabilities;
 }
 
+static inline int arch_monitor_disable_ptwalks(struct domain *d)
+{
+    d->arch.monitor.disable_ptwalks = true;
+    return 0;
+}
+
 int arch_monitor_domctl_event(struct domain *d,
                               struct xen_domctl_monitor_op *mop);
 
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 666aeb71bf..a75b731a57 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -1028,6 +1028,7 @@ struct xen_domctl_psr_cmt_op {
  * to ensure all vCPUs have resumed before it is safe to turn it off.
  */
 #define XEN_DOMCTL_MONITOR_OP_CONTROL_REGISTERS 4
+#define XEN_DOMCTL_MONITOR_OP_DISABLE_PTWALKS   5
 
 #define XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG         0
 #define XEN_DOMCTL_MONITOR_EVENT_MOV_TO_MSR            1
-- 
2.25.1