linux-kernel - Re: [PATCH v8 26/33] x86/fred: allow single-step trap and NMI when starting a new thread

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Date:   Mon, 05 Jun 2023 15:52:02 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, peterz@...radead.org, andrew.cooper3@...rix.com,
        seanjc@...gle.com, pbonzini@...hat.com, ravi.v.shankar@...el.com,
        jiangshanlai@...il.com, shan.kang@...el.com
Subject: Re: [PATCH v8 26/33] x86/fred: allow single-step trap and NMI when
 starting a new thread

On Mon, Jun 05 2023 at 15:50, Thomas Gleixner wrote:

> On Mon, Apr 10 2023 at 01:14, Xin Li wrote:
>> Allow single-step trap and NMI when starting a new thread, thus once
>> the new thread returns to ring3, single-step trap and NMI are both
>> enabled immediately.
>>
>> High-order 48 bits above the lowest 16 bit CS are discarded by the
>> legacy IRET instruction, thus can be set unconditionally, even when
>> FRED is not enabled.
>
> I assume this has been validated to be true on _all_ CPU incarnations of
> _all_ x86 vendors.

It's also ensured that VMMs do not get confused by this, right?

> If so, then please document it. If not, then go back to the drawing
> board.
>
> Thanks,
>
>         tglx