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Re: [committed, PATCH 1/3] x86: CET v2.0: Update NOTRACK prefix
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Tue, 4 Jul 2017 08:42:46 -0700
- Subject: Re: [committed, PATCH 1/3] x86: CET v2.0: Update NOTRACK prefix
- Authentication-results: sourceware.org; auth=none
- References: <20170621153503.GA6542@gmail.com> <595A0BFB0200007800167A2F@prv-mh.provo.novell.com>
On Mon, Jul 3, 2017 at 12:18 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 21.06.17 at 17:35, <email@example.com> wrote:
>> Update NOTRACK prefix handling to support memory indirect branch for
>> CET v2.0:
> It is pretty hard to judge whether the changes here are correct
> without the doc being really precise on the obvious corner case:
> How does one correctly use a segment override and a notrack
> prefix on an indirect call/jmp with a memory operand? Most
> importantly, how does one use %ds: but not notrack, notrack
> but not %ds:, or %ds and notrack?
The spec has
No-track Prefix for Near Indirect Call/Jmp
Near indirect call and jmp instructions when prefixed with 3EH are
termed “non-tracked indirect control
transfer instructions” and do not modify the CET indirect branch
tracker. Far call and jmp are always tracked
and ignore the 3EH prefix. The NO_TRACK_EN control in the
IA32_U_CET/IA32_S_CET MSR enables this
no-track prefix treatment. When this control is 0, the near indirect
call and jmp are always tracked irrespec-
tive of the presence of the 3EH prefix.
In 64-bit mode, the 3EH prefix on an indirect call or jmp is
recognized as a no-track prefix when the following
conditions are satisfied.
3EH must be the last legacy prefix of any group (except any REX).
There must not be a 64H/65H prefix on the instruction.
In legacy/compatibility mode, the 3EH prefix on an indirect call or
jmp is recognized as a no-track prefix
when it is the last group 2 prefix on the instruction.
That means you can't use the notrack pefix with a segment override
and you can't use %ds segment override on indirect branch.