Hacker News Reader

Comment by teamhappy | Hacker News Reader

The performance hit is (supposedly) much more significant on Haswell and older, which they forgot to include.

Specifically, the oldest generation they tested (Skylake) is the first to have the PCID feature that avoids the complete TLB flush on every syscall.

Continuing with their strategy of full transparency..

> (Skylake) is the first to have the PCID feature

PCID was introduced with Westmere (2010). [1]

However it wasn't used in the Linux kernel until 4.14 because no one saw the need. [2]

[1] https://www.realworldtech.com/westmere/

[2] https://stackoverflow.com/questions/20155304/does-linux-use-...

But only Haswell added INVPCID.

This is the key. Without INVPCID PCID is good as useless.

How convenient.

I own haswell and haven't felt any performance impact...

    > For context, on newer CPUs such as on Skylake and
    > beyond, Intel has refined the instructions used to
    > disable branch speculation to be more specific to
    > indirect branches, reducing the overall performance
    > penalty of the Spectre mitigation.

https://cloudblogs.microsoft.com/microsoftsecure/2018/01/09/...

Haswell and Broadwell introduced INVPCID support, unless there were further undocumented improvements I'm not entirely sure what this is based on.

Reply on news.ycombinator.com