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

[Shukla, Manali]

On 2/13/2023 6:40 PM, Peter Zijlstra wrote:
> 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 ?

Not exactly. 
Unlike exclude_guest where profiler decides whether it wants to trace
guest data or not, PreventHostIBS gives control to the Guest. Secured 
guests(SEV-ES/SEV-SNP) can disallow the use of IBS by the hypervisor,
in order to limit the information which can be gathered by host from
its execution.

-Manali