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

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Date:   Wed, 26 Jul 2017 20:43:35 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Andy Lutomirski <luto@...capital.net>
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 26/07/17 19:57, Andy Lutomirski wrote:
> 
> 
>> 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)

ENTRY() aligns on 16 byte boundary. So I have to avoid ENTRY(foo) above
in the Xen case when I want to fall through.

So either I have to do something like PV_ENTRY (I could avoid the magic
"3" by using the xen_foo entry via pv_trap_entry()), or I need the stub
with "jmp" for Xen.


Juergen