Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 29 Nov 2017 12:48:39 -0800 | Subject | Re: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling with IRQ state tracing enabled |
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On Wed, Nov 29, 2017 at 8:51 AM, David Laight <David.Laight@aculab.com> wrote: > > SWAPGS is a PITA. The hardware designers should have tried to write > all the interrupt handling code.
That's actually my biggest beef with x86-64. The kernel entry/exit is completely misdesigned.
That was actually better in the original i386, which handles nesting ok (modulo the NMI disable flag and the STI shadow bit which doesn't save/restore properly).
Yes, the 386 model of infinite indirection through IDT/GDT/TSS is broken and should have had a mode bit to just replace with a simple "use this stack and address for kernel entry" MSR register - but that simplified kernel entry/exit should have saved/restored _more_ info, not less (eg "save/restore segment shadow state rather than the descriptor number that needs the insane indirection" etc)
The problem was never the few push/pop instructions that kernel entry involved.
The entry/exit garbage admittedly started before x86-64 itself - all those 'syscall' variants are equally broken. Not saving/restoring state properly is just unbelievable sh*t. x86-64 then made things worse with SWAPGS etc.
A lot of people hate x86 because of instruction decoding. No, if you need a reason to dislike x86, it's because of exception handling and iret.
Rant over.
Linus
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