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How did they confirm that the mitigations weren't still being used? If you're still using separate page tables for the kernel, well of course it's going to rema

How did they confirm that the mitigations weren't still being used? If you're still using separate page tables for the kernel, well of course it's going to remain slow. The point of the fixed silicon is so you don't need the mitigation, not that the mitigation gets faster.

opencl 8y ago
The "fixed silicon" at least so far mostly seems to be hardware implementations of the existing software mitigation techniques with very similar performance characteristics.
- tedunangst 8y ago
  Can we test this? Can we install an unpatched OS and observe the same performance penalty? In the case of meltdown, it seems unlikely the CPU is maintaining its own shadow page table. How would it do that?
  - IntelMiner 8y ago
    You could compile a Linux kernel with the Spectre defenses disabled. I assume userspace software that includes mitgations could also be patched out in the same way for testing
  - tarlinian 8y ago
    I agree, I remember reading this article and thinking something was odd...how could you implement page table isolation in hardware? The Spectre improvement could plausibly be more similar to software mitigation...
- quotemstr 8y ago
  And that's a shame, since in principle, the hardware can do better than software. I've heard of approaches ranging from better cache partitioning to transactional commit to cache on instruction retirement. I can only imagine that Intel is working on these systemic fixes in the next big microarchitecture revision while continuing to apply cruder hacks on older cores that can't, say, alter low-level instruction retirement much.
tolien 8y ago
There's a screenshot linked to in the comments [1] showing how you can query whether the mitigations are enabled, and showing the results for a 9900k. 1: https://www.anandtech.com/comments/13659/analyzing-core-i9-9900k-performance-with-spectre-and-meltdown-hardware-mitigations/625538 https://www.anandtech.com/comments/13659/analyzing-core-i9-9... - screenshot comes from anandtech.com

opencl 8y ago

The "fixed silicon" at least so far mostly seems to be hardware implementations of the existing software mitigation techniques with very similar performance characteristics.

tedunangst 8y ago
Can we test this? Can we install an unpatched OS and observe the same performance penalty? In the case of meltdown, it seems unlikely the CPU is maintaining its own shadow page table. How would it do that?
- IntelMiner 8y ago
  You could compile a Linux kernel with the Spectre defenses disabled. I assume userspace software that includes mitgations could also be patched out in the same way for testing
- tarlinian 8y ago
  I agree, I remember reading this article and thinking something was odd...how could you implement page table isolation in hardware? The Spectre improvement could plausibly be more similar to software mitigation...
quotemstr 8y ago
And that's a shame, since in principle, the hardware can do better than software. I've heard of approaches ranging from better cache partitioning to transactional commit to cache on instruction retirement. I can only imagine that Intel is working on these systemic fixes in the next big microarchitecture revision while continuing to apply cruder hacks on older cores that can't, say, alter low-level instruction retirement much.

tolien 8y ago

There's a screenshot linked to in the comments [1] showing how you can query whether the mitigations are enabled, and showing the results for a 9900k. 1: https://www.anandtech.com/comments/13659/analyzing-core-i9-9900k-performance-with-spectre-and-meltdown-hardware-mitigations/625538 https://www.anandtech.com/comments/13659/analyzing-core-i9-9... - screenshot comes from anandtech.com