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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 29 May 2020 11:32:14 -0700
Message-ID: <CALCETrXKdh=nBWz96pow5roLmh0ez2YeQ9P+H5gxdor5TfrqUQ@mail.gmail.com>
Subject: Re: [PATCH V2 0/4] x86/entry: simply stack switching when exception
 on userspace
To:     Lai Jiangshan <laijs@linux.alibaba.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
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On Fri, May 29, 2020 at 1:26 AM Lai Jiangshan <laijs@linux.alibaba.com> 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.

--Andy