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i386/kvm: add NoNonArchitecturalCoreSharing Hyper-V enlightenment

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Hyper-V TLFS specifies this enlightenment as:
"NoNonArchitecturalCoreSharing - Indicates that a virtual processor will never
share a physical core with another virtual processor, except for virtual
processors that are reported as sibling SMT threads. This can be used as an
optimization to avoid the performance overhead of STIBP".

However, STIBP is not the only implication. It was found that Hyper-V on
KVM doesn't pass MD_CLEAR bit to its guests if it doesn't see
NoNonArchitecturalCoreSharing bit.

KVM reports NoNonArchitecturalCoreSharing in KVM_GET_SUPPORTED_HV_CPUID to
indicate that SMT on the host is impossible (not supported of forcefully
disabled).

Implement NoNonArchitecturalCoreSharing support in QEMU as tristate:
'off' - the feature is disabled (default)
'on' - the feature is enabled. This is only safe if vCPUS are properly
 pinned and correct topology is exposed. As CPU pinning is done outside
 of QEMU the enablement decision will be made on a higher level.
'auto' - copy KVM setting. As during live migration SMT settings on the
source and destination host may differ this requires us to add a migration
blocker.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20191018163908.10246-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2019-10-18 18:39:08 +02:00 • committed by Paolo Bonzini
parent f173374970
commit 30d6ff662d
5 changed files with 44 additions and 0 deletions
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13
docs/hyperv.txt
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@ -184,6 +184,19 @@ enabled.
Requires: hv-vpindex, hv-synic, hv-time, hv-stimer
3.17. hv-no-nonarch-coresharing=on/off/auto
===========================================
This enlightenment tells guest OS that virtual processors will never share a
physical core unless they are reported as sibling SMT threads. This information
is required by Windows and Hyper-V guests to properly mitigate SMT related CPU
vulnerabilities.
When the option is set to 'auto' QEMU will enable the feature only when KVM
reports that non-architectural coresharing is impossible, this means that
hyper-threading is not supported or completely disabled on the host. This
setting also prevents migration as SMT settings on the destination may differ.
When the option is set to 'on' QEMU will always enable the feature, regardless
of host setup. To keep guests secure, this can only be used in conjunction with
exposing correct vCPU topology and vCPU pinning.
4. Development features
========================