linux-kernel - Re: [RFC PATCH kernel 1/2] perf/x86/amd: Add amd_prevent_hostibs_window() to set per-cpu ibs

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Date:   Mon, 13 Feb 2023 14:10:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Manali Shukla <manali.shukla@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com, acme@...nel.org,
        jolsa@...nel.org, namhyung@...nel.org, tglx@...utronix.de,
        bp@...en8.de, dave.hansen@...ux.intel.com, seanjc@...gle.com,
        pbonzini@...hat.com, jpoimboe@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, babu.moger@....com,
        sandipan.das@....com, jmattson@...gle.com, thomas.lendacky@....com,
        nikunj@....com, ravi.bangoria@....com, eranian@...gle.com,
        irogers@...gle.com, kvm@...r.kernel.org, x86@...nel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [RFC PATCH kernel 1/2] perf/x86/amd: Add
 amd_prevent_hostibs_window() to set per-cpu ibs_flags

On Mon, Feb 06, 2023 at 06:05:44AM +0000, Manali Shukla wrote:
> Add a function to set per-cpu ibs_flags based on an active or inactive
> PreventHostIBS window.
> 
> MSR_AMD64_IBSFETCHCTL[IbsFetchEn] and MSR_AMD64_IBSOPCTL[IbsOpEn] bits
> need to be cleared for PreventHostIBS feature to be enabled before VMRUN
> is executed.
> 
> ENABLE bit and VALID bit for MSR_AMD64_IBSFETCHCTL are contained in the
> same MSR and same is the case with MSR_AMD64_IBSOPCTL.
> 
> Consider the following scenario:
> - The IBS MSR which has ENABLE bit set and VALID bit clear is read.
> - During the process of clearing the ENABLE bit and writing the IBS MSR
>   to disable IBS, an IBS event can occur that sets the VALID bit.
> - The write operation on IBS MSR can clear the newly set VALID bit.
> - Since this situation is occurring in the CLGI/STGI window
>   (PreventHostIBS window), the actual NMI is not taken.
> - Once VMRUN is issued, it will exit with VMEXIT_NMI. As soon as STGI is
>   executed, the pending NMI will trigger.
> - The IBS NMI handler checks for the VALID bit to determine if the NMI
>   is generated because of IBS.
> - Since VALID bit is now clear, it doesn't recognize that an IBS event
>   is occurred. Due to this reason, the dazed and confused unknown NMI
>   messages are generated.
> 
> amd_prevent_hostibs_window() is added to avoid these messages when
> PreventHostIBS window is active and PreventHostIBS feature is enabled
> for the guest.
> 
> Signed-off-by: Manali Shukla <manali.shukla@....com>

URGH... so am I reading this right that this is a sodding terrible
software implementation of perf_event_attr::exclude_guest ?