Fix Heartbleed demo legacy lock-up due to 32-bit mcycle read#75
Merged
AlexJones0 merged 1 commit intolowRISC:mainfrom May 22, 2025
Merged
Fix Heartbleed demo legacy lock-up due to 32-bit mcycle read#75AlexJones0 merged 1 commit intolowRISC:mainfrom
AlexJones0 merged 1 commit intolowRISC:mainfrom
Conversation
The legacy demo unintentionally aliased a 32-bit `rdcycle` function to `rdcycle64`, and used its 32-bit output as a 64-bit value. This caused an overflow in `mcycle` after a few minutes of running, which sometimes left the demo stuck and unable to progress in its legacy component. Replace this function with an actual 64-bit mcycle read using the low/high registers appropriately. Signed-off-by: Alex Jones <alex.jones@lowrisc.org>
engdoreis
approved these changes
May 22, 2025
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Note: Ideally merge after #74 to pull in the fixes necessary to get the Heartbleed demo to compile first.
The legacy demo unintentionally aliased a 32-bit
$$\frac{2^{32} - 1}{40 * 1000 * 1000} \approx 1$$ min 47 s.
rdcyclefunction tordcycle64, and used its 32-bit output as a 64-bit value. This caused an overflow in the value ofmcyclethat was read after a short while of running, which left the demo stuck and unable to progress in its legacy component. Specifically,So after around 2 minutes the legacy component of the demo will stop working.
This PR replaces this function with an actual 64-bit mcycle read using the low/high registers appropriately. It will now take >10000 years to overflow, so I've opted not to add additional logic to catch overflows here.