Re: [RFC PATCH kernel 0/2] PreventHostIBS feature for SEV-ES and SNP guests

[Manali Shukla <manali.shukla@amd.com>]

On 3/15/2023 10:33 AM, Manali Shukla wrote:
> On 2/6/2023 11:35 AM, Manali Shukla wrote:
>> Adds support for PreventHostIBS feature for SEV-ES and SNP guests.
>> Currently, the hypervisor is able to inspect instruction based samples
>> from the guest and gather execution information.  With enablement of
>> PreventHostIBS feature, SEV-ES and SNP guests may choose to disallow
>> use of instruction based sampling by the hypervisor in order to limit
>> the information gathered about their execution.  (More information in
>> Section 15.36.17 APM Volume 2)
>>
>> While implementing this feature, unknown NMIs were being seen. On
>> further investigation, a race was found effecting the IBS FETCH/OP
>> MSR.
>>
>> ENABLE bit and VALID bit for IBS_FETCH_CTL are contained in the same
>> MSR and same is the case with IBS_OP_CTL.
>>
>> 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 and 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 which in turn generates the dazed and confused unknown
>>   NMI messages.
>>
>> Per-cpu ibs_flags which indicates whether PreventHostIBS window is
>> active/inactive are added to avoid the above mentioned race.
>>
>> An active PreventHostIBS window is set before calling VMRUN and
>> cleared after STGI. PreventHostIBS window check is added to
>> perf_ibs_handle_irq(), to avoid unknown NMIs and treat them as handled
>> when window is active.
>>
>> There are 2 patches in this series.
>> 1) Add amd_prevent_hostibs_window() function to set per-cpu ibs_flags
>> based on an active/inactive PreventHostIBS window.
>> 2) Enable PreventHostIBS for SEV-ES and SNP guests.
>>
>> Testing done:
>> - Executed program symbols in guest are not captured in host when
>>   PreventHostIBS feature is enabled.
>> - Generated 1000+ NMIs using cpuid command, no unknown NMIs are seen
>>   after enablement of PreventHostIBS feature.
>>
>> Qemu commandline to enable PreventHostIBS on guest.
>>
>> qemu-system-x86_64 -enable-kvm -cpu EPYC-v4,+nohostibs \ ..
>>
>> Manali Shukla (2):
>>   perf/x86/amd: Add amd_prevent_hostibs_window() to set per-cpu
>>     ibs_flags
>>   KVM: SEV: PreventHostIBS enablement for SEV-ES and SNP guest
>>
>>  arch/x86/events/amd/ibs.c          | 64 ++++++++++++++++++++++++++++++
>>  arch/x86/include/asm/cpufeatures.h |  1 +
>>  arch/x86/include/asm/perf_event.h  | 20 ++++++++++
>>  arch/x86/kvm/svm/sev.c             | 10 +++++
>>  arch/x86/kvm/svm/svm.c             | 39 +++++++++++++++++-
>>  arch/x86/kvm/svm/svm.h             |  1 +
>>  6 files changed, 133 insertions(+), 2 deletions(-)
>>
> 
> A gentle reminder for the review.
> 
> -Manali

A gentle reminder for the review.

-Manali