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Date:   Tue, 6 Mar 2018 13:27:01 +0100
From:   Joerg Roedel <joro@8bytes.org>
To:     Brian Gerst <brgerst@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
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        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Joerg Roedel <jroedel@suse.de>
Subject: Re: [PATCH 11/34] x86/entry/32: Handle Entry from Kernel-Mode on
 Entry-Stack
Message-ID: <20180306122701.GX16484@8bytes.org>
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Hi Brian,

On Mon, Mar 05, 2018 at 11:41:01AM -0500, Brian Gerst wrote:
> We can keep the same process as the existing debug/NMI handlers -
> leave the current exception pt_regs on the entry stack and just switch
> to the task stack for the call to the handler.  Then switch back to
> the entry stack and continue.  No copying needed.

I looked into this and things are a bit more complicated than in the NMI
and debug handlers. The current code after pt_regs is set up relies on
%esp pointing to the pt_regs structure. But if pt_regs could be on
another stack we need to carry the pt_regs pointer in another register
through the whole ret_from_exception code-path until we actually switch
back the stack.

Since the code-path is used for all stack/cr3 entry/exit cases we need
to setup the extra pt_regs pointer unconditionally and update all places
that reference it through %esp.

It can certainly be done but it looks like another major surgery in the
entry code to optimize a slow-path for handling unlikely segment-loading
exceptions and debug traps. I am not sure if it's worth it.

Regards,

	Joerg