From cd68557b4c86a85cbcea44b27d5013b0afa8d1ab Mon Sep 17 00:00:00 2001
From: Jason White <jasonwhite@fb.com>
Date: Tue, 1 Mar 2022 15:50:23 -0800
Subject: [PATCH] Fix bug in check_for_xen_pmi_bug

Summary: The `count` wasn't getting initialized properly, so it was possible in theory that the check could have been failing. I noticed this while attempting to convert this chunk of `llvm_asm!()` to the new `asm!()` syntax. Also added documentation explaining what this code is doing because it's not easy to read.

Reviewed By: johnhurt

Differential Revision: D34542806

fbshipit-source-id: 0f79b9d36757756a1cf76e98830f9cc76f06336c
---
 reverie-ptrace/src/validation.rs | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/reverie-ptrace/src/validation.rs b/reverie-ptrace/src/validation.rs
index dc539be..d78f037 100644
--- a/reverie-ptrace/src/validation.rs
+++ b/reverie-ptrace/src/validation.rs
@@ -438,7 +438,8 @@ fn check_for_kvm_in_txcp_bug() -> Result<(), PmuValidationError> {
 }
 
 fn check_for_xen_pmi_bug(precise_ip: bool) -> Result<(), PmuValidationError> {
-    let mut count: i32;
+    #[allow(unused_assignments)]
+    let mut count: i32 = -1;
     let mut attr = ticks_attr(precise_ip);
     attr.__bindgen_anon_1.sample_period = NUM_BRANCHES - 1;
     let fd = start_counter(0, -1, &mut attr, None)?;
@@ -468,6 +469,31 @@ fn check_for_xen_pmi_bug(precise_ip: bool) -> Result<(), PmuValidationError> {
 
     #[allow(deprecated)]
     unsafe {
+        // The following asm block does this:
+        // ```
+        // let ret = syscall!(sys_ioctl, raw_fd, _PERF_EVENT_IOC_ENABLE, 0);
+        // if ret >= -4095 as u64 { return; }
+        // let ret = syscall!(SYS_ioctl, raw_fd, _PERF_EVENT_IOC_RESET, 0);
+        // // From this point on, all conditional branches count!
+        // if ret >= -4095 as u64 { return; }
+        // // Reset the counter period to the desired value.
+        // let ret = syscall!(SYS_ioctl, raw_fd, _PERF_EVENT_IOC_PERIOD, attr.sample_period);
+        // if ret >= -4095 as u64 { return; }
+        // let mut iterations = NUM_BRANCHES - 2;
+        // loop {
+        //     iterations -= 1;
+        //     accumulator *= 7;
+        //     accumulator += 2;
+        //     accumulator &= 0xffffff;
+        //     if iterations == 0 {
+        //         break;
+        //     }
+        // }
+        //
+        // let ret = syscall!(SYS_ioctl, raw_fd, _PERF_EVENT_IOC_DISABLE, 0);
+        // if ret >= -4095 as u64 { return; }
+        // count = 0;
+        // ```
         llvm_asm!(
             "
             mov $2, %rax;
@@ -548,7 +574,7 @@ fn check_for_xen_pmi_bug(precise_ip: bool) -> Result<(), PmuValidationError> {
         )));
     }
 
-    let has_xen_pmi_bug = (count > 0 && count as u64 > NUM_BRANCHES) || count == -1;
+    let has_xen_pmi_bug = count as u64 > NUM_BRANCHES || count == -1;
 
     if has_xen_pmi_bug {
         Err(PmuValidationError::IntelXenPmiBugDetected)