linux-kernel - Re: [PATCH v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with its per CPU cache

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Message-ID: <1D01F3E8-BEFF-4938-B538-C3657AA31742@zytor.com>
Date: Thu, 09 Jan 2025 01:46:29 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Dave Hansen <dave.hansen@...el.com>, "Xin Li (Intel)" <xin@...or.com>,
        linux-kernel@...r.kernel.org
CC: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, andrew.cooper3@...rix.com
Subject: Re: [PATCH v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with its per CPU cache

On January 8, 2025 12:14:18 PM PST, Dave Hansen <dave.hansen@...el.com> wrote:
>On 1/7/25 18:36, Xin Li (Intel) wrote:
>> +	 * Another option is to leave the FRED RSP0 MSR as-is, because the RESET
>> +	 * state of FRED MSRs is zero and INIT does not change the value of the
>> +	 * FRED MSRs in a CPU offline/online cycle.  But it doesn't seem safe to
>> +	 * depend on the properties of INIT as that's way too many things that
>> +	 * could cause bugs.
>
>Wouldn't that also open us up to kexec problems? If the last kernel
>(maybe not even Linux) left RSP0 set, then the current kernel might
>eventually try to run userspace with the old kernel's RSP0 value.
>

Yeah, it is a nonstarter. I believe Xin was trying to document the problems with *not* doing this, so someone doesn't have the bright idea to "optimize" this later.