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

From: Manali Shukla
Date: Wed Mar 15 2023 - 01:03:53 EST


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