Subject: Re: [PATCH v1] xen: get rid of paravirt op adjust_exception_frame
To: Andy Lutomirski <luto@amacapital.net>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        X86 ML <x86@kernel.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>
References: <20170724142853.26448-1-jgross@suse.com>
 <CALCETrVqUYbFiGLi1mqNq5ej5zT1cnfPk=3dwDu9D8kNFTQasw@mail.gmail.com>
From: Juergen Gross <jgross@suse.com>
Message-ID: <76679231-8596-ecef-bb11-f6cfefb39e23@suse.com>
Date: Wed, 26 Jul 2017 17:12:46 +0200
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On 26/07/17 15:48, Andy Lutomirski wrote:
> On Mon, Jul 24, 2017 at 7:28 AM, Juergen Gross <jgross@suse.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, ...)

Okay.

>>  /* 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.

Taking Andrew's comments into account I can drop it?


Juergen