linux-kernel - Re: [PATCH] x86: insn decoder: create artificial 3rd byte for 2-byte VEX

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Message-ID: <CAK1hOcNQsTqh2u2hEW+_Ps=CUH7QFWLQG=1r28ZkbJ-QfQXWjw@mail.gmail.com>
Date:	Fri, 13 Feb 2015 15:12:47 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: insn decoder: create artificial 3rd byte for 2-byte VEX

On Fri, Feb 13, 2015 at 1:32 PM, Masami Hiramatsu
<masami.hiramatsu.pt@...achi.com> wrote:
> (2015/02/13 4:04), Denys Vlasenko wrote:
>> Before this patch, users need to do this to fetch vex.vvvv:
>>
>>         if (insn->vex_prefix.nbytes == 2) {
>>                 vex_vvvv = ((insn->vex_prefix.bytes[1] >> 3) & 0xf) ^ 0xf;
>>         }
>>         if (insn->vex_prefix.nbytes == 3) {
>>                 vex_vvvv = ((insn->vex_prefix.bytes[2] >> 3) & 0xf) ^ 0xf;
>>         }
>>
>> Make it so that insn->vex_prefix.bytes[2] always contains vex.wvvvvLpp bits.
>>
>> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
>
> Looks OK for me :)
>
> Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
>
> BTW, where would you use this ??

arch/x86/kernel/uprobes.c

    reg2 = 0xff;        /* Fetch vex.vvvv */
    if (insn->vex_prefix.nbytes == 2)
        reg2 = insn->vex_prefix.bytes[1];
    else if (insn->vex_prefix.nbytes == 3)
        reg2 = insn->vex_prefix.bytes[2];


https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/uprobes.c#n354
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