Re: [PATCH V2 0/4] x86/entry: simply stack switching when exception on userspace

From: Andy Lutomirski
Date: Fri May 29 2020 - 14:32:29 EST

On Fri, May 29, 2020 at 1:26 AM Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx> wrote:
> 7f2590a110b8("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
> has resulted that when exception on userspace, the kernel (error_entry)
> always push the pt_regs to entry stack(sp0), and then copy them to the
> kernel stack.
> And recent x86/entry work makes interrupt also use idtentry
> and makes all the interrupt code save the pt_regs on the sp0 stack
> and then copy it to the thread stack like exception.
> This is hot path (page fault, ipi), such overhead should be avoided.
> And the original interrupt_entry directly switches to kernel stack
> and pushes pt_regs to kernel stack. We should do it for error_entry.
> This is the job of patch1.
> Patch 2-4 simply stack switching for .Lerror_bad_iret by just doing
> all the work in one function (fixup_bad_iret()).
> The patch set is based on tip/x86/entry (28447ea41542) (May 20).

There are definitely good cleanups in here, but I think it would be
nice rebased to whatever lands in 5.8-rc1 settles.