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

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Message-ID: <CALCETrVqUYbFiGLi1mqNq5ej5zT1cnfPk=3dwDu9D8kNFTQasw@mail.gmail.com>
Date:   Wed, 26 Jul 2017 06:48:42 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Juergen Gross <jgross@...e.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        X86 ML <x86@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v1] xen: get rid of paravirt op adjust_exception_frame

On Mon, Jul 24, 2017 at 7:28 AM, Juergen Gross <jgross@...e.com> wrote:
> When running as Xen pv-guest the exception frame on the stack contains
> %r11 and %rcx additional to the other data pushed by the processor.
>
> Instead of having a paravirt op being called for each exception type
> prepend the Xen specific code to each exception entry. When running as
> Xen pv-guest just use the exception entry with prepended instructions,
> otherwise use the entry without the Xen specific code.

I think this is a nice cleanup, but I'm wondering if it would be even
nicer if the Xen part was kept out-of-line.  That is, could Xen have
little stubs like:

xen_alignment_check:
  pop %rcx
  pop %r11
  jmp alignment_check

rather than using the macros in entry_64.S that you have?  Then you
could adjust set_trap_gate instead of pack_gate and maybe even do
something like:

#define set_trap_gate(..., name, ...) set_native_or_xen_trap_gate(...,
name, xen_##name, ...)

>  /* 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.


--Andy