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Subject: Re: [PATCH 1/2] x86/entry/64: Do not clear %rbx under Xen
To:     "M. Vefa Bicakci" <m.v.b@runbox.com>, linux-kernel@vger.kernel.org
Cc:     Dominik Brodowski <linux@dominikbrodowski.net>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Juergen Gross <jgross@suse.com>,
        xen-devel@lists.xenproject.org, x86@kernel.org,
        stable@vger.kernel.org
References: <20180721194909.23903-1-m.v.b@runbox.com>
 <5a21b844-36a9-8fe4-c6cd-6fade3dfa941@oracle.com>
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <15aa906e-0e42-31ce-f3d3-167d263e9226@oracle.com>
Date:   Mon, 23 Jul 2018 10:56:28 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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On 07/21/2018 07:18 PM, M. Vefa Bicakci wrote:
> On 07/21/2018 05:19 PM, Boris Ostrovsky wrote:
>> On 07/21/2018 03:49 PM, M. Vefa Bicakci wrote:
>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>> index c7449f377a77..96e8ff34129e 100644
>>> --- a/arch/x86/entry/entry_64.S
>>> +++ b/arch/x86/entry/entry_64.S
>>> @@ -1129,7 +1129,7 @@ ENTRY(xen_failsafe_callback)
>>> ????? addq??? $0x30, %rsp
>>> ????? UNWIND_HINT_IRET_REGS
>>> ????? pushq??? $-1 /* orig_ax = -1 => not a system call */
>>> -??? PUSH_AND_CLEAR_REGS
>>> +??? PUSH_AND_CLEAR_REGS clear_rbx=0
>>
>>
>> Do we need this at all? We are returning from the hypervisor here.
>>
>> -boris
>>
>>> ????? ENCODE_FRAME_POINTER
>>> ????? jmp??? error_exit
>>> ? END(xen_failsafe_callback)
>
> Hello Boris,
>
> If you are referring to the PUSH_AND_CLEAR_REGS macro itself, I am not
> sure;
> however, not clearing the RBX register seemed to resolve the issues
> mentioned
> in the commit message for me. Given Andy's comment though, I believe
> that the
> approach in this patch may not be correct.


I was only referring to register clearing part of PUSH_AND_CLEAR_REGS.


-boris