linux-kernel - Re: WARNING: can't access registers at asm_common

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Message-ID: <f105a63d-6b51-3afb-83e0-e899ea40813e@suse.com>
Date:   Mon, 16 Nov 2020 12:56:32 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        X86 ML <x86@...nel.org>
Subject: Re: WARNING: can't access registers at asm_common_interrupt

On 13.11.20 18:34, Andy Lutomirski wrote:
> On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
> <andrew.cooper3@...rix.com> wrote:
>>
>> On 11/11/2020 20:15, Josh Poimboeuf wrote:
>>> On Wed, Nov 11, 2020 at 09:07:30PM +0100, Peter Zijlstra wrote:
>>>> On Wed, Nov 11, 2020 at 01:59:00PM -0600, Josh Poimboeuf wrote:
>>>>> On Wed, Nov 11, 2020 at 08:42:06PM +0100, Peter Zijlstra wrote:
>>>>>>> Would objtool have an easier time coping if this were implemented in
>>>>>>> terms of a static call?
>>>>>> I doubt it, the big problem is that there is no visibility into the
>>>>>> actual alternative text. Runtime patching fragments into static call
>>>>>> would have the exact same problem.
>>>>>>
>>>>>> Something that _might_ maybe work is trying to morph the immediate
>>>>>> fragments into an alternative. That is, instead of this:
>>>>>>
>>>>>> static inline notrace unsigned long arch_local_save_flags(void)
>>>>>> {
>>>>>>     return PVOP_CALLEE0(unsigned long, irq.save_fl);
>>>>>> }
>>>>>>
>>>>>> Write it something like:
>>>>>>
>>>>>> static inline notrace unsigned long arch_local_save_flags(void)
>>>>>> {
>>>>>>     PVOP_CALL_ARGS;
>>>>>>     PVOP_TEST_NULL(irq.save_fl);
>>>>>>     asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
>>>>>>                                     "PUSHF; POP _ASM_AX",
>>>>>>                                     X86_FEATURE_NATIVE)

I am wondering whether we really want a new feature (basically "not
XENPV). We could use ~X86_FEATURE_XENPV and teach apply_alternatives()
to understand negated features (yes, this limits the number of features
to 32767, but I don't think this is a real problem for quite some time).

Thoughts?


Juergen

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