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

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Message-Id: <3487CD8D-4BC6-46D1-861A-A6FF31EB6087@amacapital.net>
Date:   Wed, 26 Jul 2017 13:57:42 -0400
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 Jul 26, 2017, at 11:50 AM, Juergen Gross <jgross@...e.com> wrote:
> 
>> On 26/07/17 15:48, Andy Lutomirski wrote:
>>> 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, ...)
> 
> I think I'll have something like:
> 
> #define pv_trap_entry(name) (xen_pv_domain() ? xen_ ## name : name)
> 
> and use it like:
> 
> set_intr_gate(X86_TRAP_AC, pv_trap_entry(alignment_check));
> 
> This will avoid having to define macros for all variants of
> set_intr_gate(), e.g. set_intr_gate_ist(), set_system_intr_gate().
> 
> Do you have any objections?
> 

Sounds good to me.

FWIW, I have no real objection to putting the Xen entry right before the native entry and falling through.  I don't love the ip -= 3 bit, though, and I think that the PV_ENTRY macro is too magical.

This might be okay, though:

XEN_PV_ENTRY_FALLTHROUGH(foo)
ENTRY(foo)
  code here



> 
> Juergen