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

[Manali Shukla]

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