linux-kernel - Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

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Message-ID: <5f62f099-c773-ae64-d685-f6db77406020@oracle.com>
Date:   Tue, 17 Oct 2017 16:59:41 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Mike Galbraith <efault@....de>, xen-devel@...ts.xenproject.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Sasha Levin <alexander.levin@...izon.com>,
        Chris Wright <chrisw@...s-sol.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        live-patching@...r.kernel.org, Alok Kataria <akataria@...are.com>,
        virtualization@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives
 infrastructure

On 10/17/2017 04:50 PM, Josh Poimboeuf wrote:
> On Tue, Oct 17, 2017 at 04:36:00PM -0400, Boris Ostrovsky wrote:
>> On 10/17/2017 04:17 PM, Josh Poimboeuf wrote:
>>> On Tue, Oct 17, 2017 at 11:36:57AM -0400, Boris Ostrovsky wrote:
>>>> On 10/17/2017 10:36 AM, Josh Poimboeuf wrote:
>>>>> Maybe we can add a new field to the alternatives entry struct which
>>>>> specifies the offset to the CALL instruction, so apply_alternatives()
>>>>> can find it.
>>>> We'd also have to assume that the restore part of an alternative entry
>>>> is the same size as the save part. Which is true now.
>>> Why?
>>>
>> Don't you need to know the size of the instruction without save and
>> restore part?
>>
>> + if (a->replacementlen == 6 && *insnbuf == 0xff && *(insnbuf+1) == 0x15)
>>
>> Otherwise you'd need another field for the actual instruction length.
> If we know where the CALL instruction starts, and can verify that it
> starts with "ff 15", then we know the instruction length: 6 bytes.
> Right?
>

Oh, OK. Then you shouldn't need a->replacementlen test(s?) in
apply_alternatives()?

-boris