[Xen-devel] [PATCH v6 1/5] x86: allow reading MSR_IA32_TSC with XENPF_resource_op

[Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>]

Memory bandwidth monitoring requires system time information returned
along with the monitoring counter to verify the correctness of the
counter value and to calculate the time elapsed between two samplings.
Add MSR_IA32_TSC to the read path and it returns scaled system time(ns)
instead of raw timestamp to elimanate the needs to convert. Also increase
RESOURCE_ACCESS_MAX_ENTRIES to 3 so MSR_IA32_TSC can be used.

Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
---
Changes in v6:
1. Latch read and IRQ disable for MSR_IA32_TSC.
2. Add comments.
---
 xen/arch/x86/platform_hypercall.c | 37 ++++++++++++++++++++++++++++++++++---
 xen/include/public/platform.h     | 10 ++++++++++
 2 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/platform_hypercall.c 
b/xen/arch/x86/platform_hypercall.c
index 32f39b2..597c723 100644
--- a/xen/arch/x86/platform_hypercall.c
+++ b/xen/arch/x86/platform_hypercall.c
@@ -61,7 +61,7 @@ long cpu_down_helper(void *data);
 long core_parking_helper(void *data);
 uint32_t get_cur_idle_nums(void);
 
-#define RESOURCE_ACCESS_MAX_ENTRIES 2
+#define RESOURCE_ACCESS_MAX_ENTRIES 3
 struct xen_resource_access {
     unsigned int nr_done;
     unsigned int nr_entries;
@@ -75,6 +75,7 @@ static bool_t allow_access_msr(unsigned int msr)
     /* MSR for CMT, refer to chapter 17.14 of Intel SDM. */
     case MSR_IA32_CMT_EVTSEL:
     case MSR_IA32_CMT_CTR:
+    case MSR_IA32_TSC:
         return 1;
     }
 
@@ -124,6 +125,7 @@ static void resource_access(void *info)
 {
     struct xen_resource_access *ra = info;
     unsigned int i;
+    u64 tsc = 0;
 
     for ( i = 0; i < ra->nr_done; i++ )
     {
@@ -133,10 +135,39 @@ static void resource_access(void *info)
         switch ( entry->u.cmd )
         {
         case XEN_RESOURCE_OP_MSR_READ:
-            ret = rdmsr_safe(entry->idx, entry->val);
+            if ( unlikely(entry->idx == MSR_IA32_TSC) ) {
+                /* Return scaled time instead of raw timestamp */
+                entry->val = get_s_time_fixed(tsc);
+                ret = 0;
+            }
+            else
+            {
+                unsigned long irqflags;
+                /*
+                 * If next entry is MSR_IA32_TSC read, then the actual rdtscll
+                 * is performed together with current entry, with IRQ disabled.
+                 */
+                bool_t read_tsc = (i < ra->nr_done - 1 &&
+                                   unlikely(entry[1].idx == MSR_IA32_TSC &&
+                                   entry[1].u.cmd == 
XEN_RESOURCE_OP_MSR_READ));
+
+                if ( unlikely(read_tsc) )
+                    local_irq_save(irqflags);
+
+                ret = rdmsr_safe(entry->idx, entry->val);
+
+                if ( unlikely(read_tsc) )
+                {
+                    rdtscll(tsc);
+                    local_irq_restore(irqflags);
+                }
+            }
             break;
         case XEN_RESOURCE_OP_MSR_WRITE:
-            ret = wrmsr_safe(entry->idx, entry->val);
+            if ( unlikely(entry->idx == MSR_IA32_TSC) )
+                ret = -EPERM;
+            else
+                ret = wrmsr_safe(entry->idx, entry->val);
             break;
         default:
             BUG();
diff --git a/xen/include/public/platform.h b/xen/include/public/platform.h
index 5c57615..3e340b4 100644
--- a/xen/include/public/platform.h
+++ b/xen/include/public/platform.h
@@ -540,6 +540,16 @@ DEFINE_XEN_GUEST_HANDLE(xenpf_core_parking_t);
 #define XEN_RESOURCE_OP_MSR_READ  0
 #define XEN_RESOURCE_OP_MSR_WRITE 1
 
+/*
+ * Specially handled MSRs:
+ * - MSR_IA32_TSC
+ * READ: Returns the scaled system time(ns) instead of raw timestamp. In
+ *       multiple entry case, if other MSR read is followed by a MSR_IA32_TSC
+ *       read, then both reads are guaranteed to be performed atomically (with
+ *       IRQ disabled). The return time indicates the point of reading that 
MSR.
+ * WRITE: Not supported.
+ */
+
 struct xenpf_resource_entry {
     union {
         uint32_t cmd;   /* IN: XEN_RESOURCE_OP_* */
-- 
1.9.1


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