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Computer Science 2021
Reasons for transient execution
Out-of-order execution
CPU starts executing instructions out of order, when their input
data is ready
Late recognition of exceptions
A CPU may not decide that an instruction will raise an exception
until it is retired
Branch prediction
CPU guesses which side of a branch will be taken, and starts
speculatively executing it before the branch condition is evaluated
Computer Science 2021
Transient execution and memory
Transient execution includes speculative loads from
Important for performance, similar to pre-fetching
Transient stores generally not sent outside the processor
Less important for performance, since stores don’t have many
Stores will be buffered and sent to cache and memory on
Transient stores can affect transient loads via store-to-load
Exceptions from transient accesses are ignored
Computer Science 2021
Announcements intermission 1: HA5
Our release of HA5 had an inconsistency that made the
performance scores look worse than they should have
Driver was set up for -O2, but Makefile had no optimization
Grading will still be based on no optimization
Instructions and driver.py have been updated
If you already have or still want to improve performance
up to the mistaken goal, up to 10 extra credit points are
Baseline (0/55)
Full credit (55/55)
Extra credit (65/55)
24,624 KCEST
1244 KCEST
10,985 KCEST
Computer Science 2021
Announcements 2: lecture Wednesday
Lecture Wednesday will be a review session for the final
Outline of material, discussion of rules and style of the exam, etc.
We will also do course evaluations
Think about things I or the TAs did that were good or bad for your
learning, and what we might do differently next time
Bring a blue/black pen or a pencil to fill out forms
Evaluations are important as we continue to explore how to make
the course better, so thanks for spending time on them
Computer Science 2021
Side and covert channels
Transient execution
Transient execution and kernel isolation: Meltdown
Transient execution and software checks: Spectre
Fixes, lessons learned, and the future
Computer Science 2021
Kernel and user memory
In many recent systems, kernel and user memory live in
the same virtual address space
E.g., x86-32 Linux used low 3GB for user, top 1G for kernel
Makes it easier for kernel to transfer data to/from processes
No need to flush the TLB when making a system call
Kernel/user separation still important for security
Kernel can hold system secrets, and other users’ data
A bit in the page table entry (“U/S” on x86) distinguishes
which pages are for kernel only
Attempted access to kernel data from user program leads to page

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