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Date:   Thu, 1 Nov 2018 15:31:07 -0400
From:   Rich Felker <dalias@libc.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jann Horn <jannh@google.com>, luto@kernel.org,
        dave.hansen@linux.intel.com, sean.j.christopherson@intel.com,
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        Ingo Molnar <mingo@redhat.com>, bp@alien8.de,
        carlos@redhat.com, adhemerval.zanella@linaro.org
Subject: Re: RFC: userspace exception fixups
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On Thu, Nov 01, 2018 at 12:10:35PM -0700, Linus Torvalds wrote:
> On Thu, Nov 1, 2018 at 11:52 AM Rich Felker <dalias@libc.org> wrote:
> >
> > There's no need to chain if the handler is specific to the context
> > where the fault happens. You just replace the handler with the one
> > relevant to the code you're about to run before you run it.
> 
> That's much too expensive to do as a system call.

See my other emails in this thread. You would register the *address*
(in TLS) of a function pointer object pointing to the handler, rather
than the function address of the handler. Then switching handler is
just a single store in userspace, no syscalls involved.

Rich