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Subject: Re: WARNING: can't access registers at asm_common_interrupt
To:     Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        X86 ML <x86@kernel.org>
References: <20201106060414.edtcb7nrbzm4a32t@shindev.dhcp.fujisawa.hgst.com>
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From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
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Date:   Mon, 16 Nov 2020 12:56:32 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
To: Andy Lutomirski <luto@kernel.org>,
 Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
 Peter Zijlstra <peterz@infradead.org>,
 Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 Nicholas Piggin <npiggin@gmail.com>, Damien Le Moal <Damien.LeMoal@wdc.com>,
 X86 ML <x86@kernel.org>
Message-ID: <f105a63d-6b51-3afb-83e0-e899ea40813e@suse.com>
Subject: Re: WARNING: can't access registers at asm_common_interrupt
References: <20201106060414.edtcb7nrbzm4a32t@shindev.dhcp.fujisawa.hgst.com>
 <20201111170536.arx2zbn4ngvjoov7@treble>
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On 13.11.20 18:34, Andy Lutomirski wrote:
> On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
> <andrew.cooper3@citrix.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 th=
e
>>>>>> actual alternative text. Runtime patching fragments into static ca=
ll
>>>>>> 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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