Re: [PATCH V3 0/3] x86/entry: simply stack switching when exception on userspace

[Lai Jiangshan <laijs@linux.alibaba.com>]

Deeply sorry, the cover-letter was forgotten to send to LKML.

Here it is:

On 2020/8/17 14:23, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> 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 2-3 simplify stack switching for .Lerror_bad_iret by just doing
> all the work in one function (fixup_bad_iret()).
> 
> The patch set is based on v5.9-rc1
> 
> Changed from V1:
> 	based on tip/master -> based on tip/x86/entry
> 
> 	patch 1 replaces the patch1,2 of V1, it borrows the
> 	original interrupt_entry's code into error_entry.
> 
> 	patch2-4 is V1's patch3-5, unchanged (but rebased)
> 
> Changed from V2:
> 	(re-)based on v5.9-rc1
> 	drop the patch4 of V2 patchset
> 
> Cc: Andy Lutomirski <luto@kernel.org>,
> Cc: Thomas Gleixner <tglx@linutronix.de>,
> Cc: Ingo Molnar <mingo@redhat.com>,
> Cc: Borislav Petkov <bp@alien8.de>,
> Cc: x86@kernel.org,
> Cc: "H. Peter Anvin" <hpa@zytor.com>,
> Cc: Peter Zijlstra <peterz@infradead.org>,
> Cc: Alexandre Chartre <alexandre.chartre@oracle.com>,
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
> Cc: Jann Horn <jannh@google.com>,
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> 
> 
> Lai Jiangshan (3):
>    x86/entry: avoid calling into sync_regs() when entering from userspace
>    x86/entry: directly switch to kernel stack when .Lerror_bad_iret
>    x86/entry: remove unused sync_regs()
> 
>   arch/x86/entry/entry_64.S    | 52 +++++++++++++++++++++++-------------
>   arch/x86/include/asm/traps.h |  1 -
>   arch/x86/kernel/traps.c      | 22 +++------------
>   3 files changed, 38 insertions(+), 37 deletions(-)
>