Mostly tons of speculative state that needs to be unwound, combined with spectre mitigations, plus tons of committed state that the interrupt prologue needs to save, plus a huge song and dance to do that correctly (that FRED should help with).
All combined with the fact that there's a good chance the memory the interrupt handler is going to touch isn't in the cached working set anymore, both in the actual L* caches and in subtler places like the branch predictors and TLBs.
alt Hacker News
Mostly tons of speculative state that needs to be unwound, combined with spectre mitigations, plus tons of committed state that the interrupt prologue needs to save, plus a huge song and dance to do that correctly (that FRED should help with).
All combined with the fact that there's a good chance the memory the interrupt handler is going to touch isn't in the cached working set anymore, both in the actual L* caches and in subtler places like the branch predictors and TLBs.