linux-kernel - Re: [Xen-devel] [PATCH v1] xen: get rid of paravirt op adjust_exception

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Date:   Wed, 26 Jul 2017 15:01:01 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Juergen Gross <jgross@...e.com>
CC:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH v1] xen: get rid of paravirt op
 adjust_exception_frame

On 26/07/17 14:48, Andy Lutomirski wrote:
>
>>  /* Runs on exception stack */
>> -ENTRY(nmi)
>> -       /*
>> -        * Fix up the exception frame if we're on Xen.
>> -        * PARAVIRT_ADJUST_EXCEPTION_FRAME is guaranteed to push at most
>> -        * one value to the stack on native, so it may clobber the rdx
>> -        * scratch slot, but it won't clobber any of the important
>> -        * slots past it.
>> -        *
>> -        * Xen is a different story, because the Xen frame itself overlaps
>> -        * the "NMI executing" variable.
>> -        */
> I would keep this comment.  The Xen frame really is in the way AFAICT.

(For reasons best explained by the original authors) there is only ever
a single stack which a PV guest registers with Xen, which functions
equivalently to tss.sp0.  There is no support for stack switching via
task switch or IST.

Therefore, nested NMIs won't clobber the top of this stack.

~Andrew