linux-kernel - Re: [PATCH v12 19/37] x86/fred: Update MSR_IA32_FRED

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Message-ID: <78A17F3A-B373-4277-8D72-A47CA84E29D8@zytor.com>
Date:   Mon, 13 Nov 2023 13:44:11 -0500
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Xin Li <xin3.li@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        luto@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        peterz@...radead.org, jgross@...e.com, ravi.v.shankar@...el.com,
        mhiramat@...nel.org, andrew.cooper3@...rix.com,
        jiangshanlai@...il.com, nik.borisov@...e.com
Subject: Re: [PATCH v12 19/37] x86/fred: Update MSR_IA32_FRED_RSP0 during task switch

On November 13, 2023 1:29:47 PM EST, Borislav Petkov <bp@...en8.de> wrote:
>On Mon, Nov 13, 2023 at 12:36:04PM -0500, H. Peter Anvin wrote:
>> A resource cannot be consumed after the value has been written; this
>> is the only necessary level of serialization, equivalent to, say, RAX.
>
>Lemme see if I understand this correctly using this context as an
>example: after this MSR_IA32_FRED_RSP0 write, any FRED events determined
>to be delivered to level 0 will use this new task stack ptr?
>
>And since the new task is not running yet and the old one isn't running
>either, we're fine here. So the "serialization point" I was talking
>about above is bollocks.
>
>Close? :)
>
>> A serializing instruction stops the entire pipeline until everything
>> has retired and any stores have become globally visible.
>
>Right, we don't need that here.
>
>Thx.
>

Yep!