Virtual NMI feature [LWN.net]

From:		Santosh Shukla <santosh.shukla-AT-amd.com>
To:		Paolo Bonzini <pbonzini-AT-redhat.com>
Subject:		[PATCHv4 0/8] Virtual NMI feature
Date:		Mon, 29 Aug 2022 15:38:42 +0530
Message-ID:		<20220829100850.1474-1-santosh.shukla@amd.com>
Cc:		Sean Christopherson <seanjc-AT-google.com>, Vitaly Kuznetsov <vkuznets-AT-redhat.com>, Jim Mattson <jmattson-AT-google.com>, Joerg Roedel <joro-AT-8bytes.org>, Tom Lendacky <thomas.lendacky-AT-amd.com>, <kvm-AT-vger.kernel.org>, <linux-kernel-AT-vger.kernel.org>, <santosh.shukla-AT-amd.com>, <mlevitsk-AT-redhat.com>, <mail-AT-maciej.szmigiero.name>
Archive-link:		Article


Change History:

v4 (v6.0-rc3):
05 - added nmi_l1_to_l2 check (Review comment from Maciej).

v3 (rebased on eb555cb5b794f):
https://lore.kernel.org/all/20220810061226.1286-1-santosh...

v2:
https://lore.kernel.org/lkml/20220709134230.2397-7-santos...

v1:
https://lore.kernel.org/all/20220602142620.3196-1-santosh...

Description:
Currently, NMI is delivered to the guest using the Event Injection
mechanism [1]. The Event Injection mechanism does not block the delivery
of subsequent NMIs. So the Hypervisor needs to track the NMI delivery
and its completion(by intercepting IRET) before sending a new NMI.

Virtual NMI (VNMI) allows the hypervisor to inject the NMI into the guest
w/o using Event Injection mechanism meaning not required to track the
guest NMI and intercepting the IRET. To achieve that,
VNMI feature provides virtualized NMI and NMI_MASK capability bits in
VMCB intr_control -
V_NMI(11) - Indicates whether a virtual NMI is pending in the guest.
V_NMI_MASK(12) - Indicates whether virtual NMI is masked in the guest.
V_NMI_ENABLE(26) - Enables the NMI virtualization feature for the guest.

When Hypervisor wants to inject NMI, it will set V_NMI bit, Processor will
clear the V_NMI bit and Set the V_NMI_MASK which means the Guest is
handling NMI, After the guest handled the NMI, The processor will clear
the V_NMI_MASK on the successful completion of IRET instruction
Or if VMEXIT occurs while delivering the virtual NMI.

If NMI virtualization enabled and NMI_INTERCEPT bit is unset
then HW will exit with #INVALID exit reason.

To enable the VNMI capability, Hypervisor need to program
V_NMI_ENABLE bit 1.

The presence of this feature is indicated via the CPUID function
0x8000000A_EDX[25].

Testing -
* Used qemu's `inject_nmi` for testing.
* tested with and w/o AVIC case.
* tested with kvm-unit-test
* tested with vGIF enable and disable.
* tested nested env:
  - L1+L2 using vnmi
  - L1 using vnmi and L2 not


Thanks,
Santosh
[1] https://www.amd.com/system/files/TechDocs/40332.pdf - APM Vol2,
ch-15.20 - "Event Injection".

Santosh Shukla (8):
  x86/cpu: Add CPUID feature bit for VNMI
  KVM: SVM: Add VNMI bit definition
  KVM: SVM: Add VNMI support in get/set_nmi_mask
  KVM: SVM: Report NMI not allowed when Guest busy handling VNMI
  KVM: SVM: Add VNMI support in inject_nmi
  KVM: nSVM: implement nested VNMI
  KVM: nSVM: emulate VMEXIT_INVALID case for nested VNMI
  KVM: SVM: Enable VNMI feature

 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/include/asm/svm.h         |  7 +++
 arch/x86/kvm/svm/nested.c          | 32 ++++++++++++++
 arch/x86/kvm/svm/svm.c             | 44 ++++++++++++++++++-
 arch/x86/kvm/svm/svm.h             | 68 ++++++++++++++++++++++++++++++
 5 files changed, 151 insertions(+), 1 deletion(-)

-- 
2.25.1