linux-kernel - Re: [PATCH] x86-64/Xen: fix stack switching

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Message-ID: <CALCETrX=SM=YBSpRv53Qmxk2PkHcTFWWvw-JGOh0C4UpvTReAA@mail.gmail.com>
Date:   Tue, 08 May 2018 02:38:25 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Lutomirski <luto@...nel.org>,
        xen-devel@...ts.xenproject.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86-64/Xen: fix stack switching

On Mon, May 7, 2018 at 5:16 AM Jan Beulich <JBeulich@...e.com> wrote:

> While on native entry into the kernel happens on the trampoline stack,
> PV Xen kernels are being entered with the current thread stack right
> away. Hence source and destination stacks are identical in that case,
> and special care is needed.

> Other than in sync_regs() the copying done on the INT80 path as well as
> on the NMI path itself isn't NMI / #MC safe, as either of these events
> occurring in the middle of the stack copying would clobber data on the
> (source) stack. (Of course, in the NMI case only #MC could break
> things.)

I think I'd rather fix this by changing the stack switch code or
alternativing around it on non-stack-switching kernels.  Or make Xen use a
trampoline stack just like native.


> I'm not altering the similar code in interrupt_entry(), as that code
> path is unreachable when running an PV Xen guest afaict.

> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> Cc: stable@...nel.org
> ---
> There would certainly have been the option of using alternatives
> patching, but afaict the patching code isn't NMI / #MC safe, so I'd
> rather stay away from patching the NMI path. And I thought it would be
> better to use similar code in both cases.

I would hope we do the patching before we enable any NMIs.


> Another option would be to make the Xen case match the native one, by
> going through the trampoline stack, but to me this would look like extra
> overhead for no gain.

Avoiding even more complexity in the nmi code seems like a big gain to me.