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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Xin Li <xin3.li@intel.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, kvm@vger.kernel.org
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com, peterz@infradead.org, andrew.cooper3@citrix.com,
        seanjc@google.com, pbonzini@redhat.com, ravi.v.shankar@intel.com,
        jiangshanlai@gmail.com, shan.kang@intel.com
Subject: Re: [PATCH v8 26/33] x86/fred: allow single-step trap and NMI when
 starting a new thread
In-Reply-To: <20230410081438.1750-27-xin3.li@intel.com>
References: <20230410081438.1750-1-xin3.li@intel.com>
 <20230410081438.1750-27-xin3.li@intel.com>
Date:   Mon, 05 Jun 2023 15:50:55 +0200
Message-ID: <877csi6mrk.ffs@tglx>
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List-ID: <kvm.vger.kernel.org>
X-Mailing-List: kvm@vger.kernel.org

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.

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

Thanks,

        tglx