Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

[Linus Torvalds]

On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@xxxxxxxxx> wrote:
>
> I've been spending some cycles on the XPFO patch set this week. For the
> patch set as it was posted for v4.13, the performance overhead of
> compiling a Linux kernel is ~40% on x86_64[1]. The overhead comes almost
> completely from TLB flushing. If we can live with stale TLB entries
> allowing temporary access (which I think is reasonable), we can remove
> all TLB flushing (on x86). This reduces the overhead to 2-3% for
> kernel compile.

I have to say, even 2-3% for a kernel compile sounds absolutely horrendous.

Kernel bullds are 90% user space at least for me, so a 2-3% slowdown
from a kernel is not some small unnoticeable thing.

           Linus