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Fix bug in check_for_xen_pmi_bug

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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
This commit is contained in:
Jason White 2022-03-01 15:50:23 -08:00 committed by Facebook GitHub Bot
parent c2d7dc125f
commit cd68557b4c
1 changed files with 28 additions and 2 deletions
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30
reverie-ptrace/src/validation.rs
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@ -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)