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

[Peter Zijlstra]

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@xxxxxxx>

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