[RFC v2 00/17] AMD: Add Secure AVIC Guest Support

Neeraj Upadhyay
2025-02-26T09:05:43Z

Introduction
------------

Secure AVIC is a new hardware feature in the AMD64 architecture to
allow SEV-SNP guests to prevent hypervisor from generating unexpected
interrupts to a vCPU or otherwise violate architectural assumptions
around APIC behavior.

One of the significant differences from AVIC or emulated x2APIC is that
Secure AVIC uses a guest-owned and managed APIC backing page. It also
introduces additional fields in both the VMCB and the Secure AVIC backing
page to aid the guest in limiting which interrupt vectors can be injected
into the guest.

Guest APIC Backing Page
-----------------------
Each vCPU has a guest-allocated APIC backing page of size 4K, which
maintains APIC state for that vCPU. The x2APIC MSRs are mapped at
their corresposing x2APIC MMIO offset within the guest APIC backing
page. All x2APIC accesses by guest or Secure AVIC hardware operate
on this backing page. The backing page should be pinned and NPT entry
for it should be always mapped while the corresponding vCPU is running.

MSR Accesses
------------
Secure AVIC only supports x2APIC MSR accesses. xAPIC MMIO offset based
accesses are not supported.

Some of the MSR accesses such as ICR writes (with shorthand equal to
self), SELF_IPI, EOI, TPR writes are accelerated by Secure AVIC
hardware. Other MSR accesses generate a #VC exception. The #VC
exception handler reads/writes to the guest APIC backing page.
As guest APIC backing page is accessible to the guest, the Secure
AVIC driver code optimizes APIC register access by directly
reading/writing to the guest APIC backing page (instead of taking
the #VC exception route).

In addition to the architected MSRs, following new fields are added to
the guest APIC backing page which can be modified directly by the
guest:

a. ALLOWED_IRR

ALLOWED_IRR vector indicates the interrupt vectors which the guest
allows the hypervisor to send. The combination of host-controlled
REQUESTED_IRR vectors (part of VMCB) and ALLOWED_IRR is used by
hardware to update the IRR vectors of the Guest APIC backing page.

#Offset #bits Description
204h 31:0 Guest allowed vectors 0-31
214h 31:0 Guest allowed vectors 32-63
...
274h 31:0 Guest allowed vectors 224-255

ALLOWED_IRR is meant to be used specifically for vectors that the
hypervisor is allowed to inject, such as device interrupts. Interrupt
vectors used exclusively by the guest itself (like IPI vectors) should
not be allowed to be injected into the guest for security reasons.

b. NMI Request

#Offset #bits Description
278h 0 Set by Guest to request Virtual NMI

LAPIC Timer Support
-------------------
LAPIC timer is emulated by hypervisor. So, APIC_LVTT, APIC_TMICT and
APIC_TDCR, APIC_TMCCT APIC registers are not read/written to the guest
APIC backing page and are communicated to the hypervisor using SVM_EXIT_MSR
VMGEXIT.

IPI Support
-----------
Only SELF_IPI is accelerated by Secure AVIC hardware. Other IPIs require
writing (from the Secure AVIC driver) to the IRR vector of the target CPU
backing page and then issuing VMGEXIT for the hypervisor to notify the
target vCPU.

KEXEC Support
-------------
Secure AVIC enabled guest can kexec to another kernel which has Secure
AVIC enabled, as the Hypervisor has Secure AVIC feature bit set in the
sev_status.

Driver Implementation Open Points
---------------------------------

The Secure AVIC driver only supports physical destination mode. If
logical destination mode need to be supported, then a separate x2apic
driver would be required for supporting logical destination mode.

Setting of ALLOWED_IRR vectors is done from vector.c for IOAPIC and MSI
interrupts. ALLOWED_IRR vector is not cleared when an interrupt vector
migrates to different CPU. Using a cleaner approach to manage and
configure allowed vectors needs more work.

Testing
-------

This series is based on top of commit 0f966b199269 "Merge branch into tip/master:
'x86/platform'" of tip/tip master branch.

Host Secure AVIC support patch series is at [1].

Qemu support patch is at [2].

QEMU commandline for testing Secure AVIC enabled guest:

qemu-system-x86_64 <...> -object sev-snp-guest,id=sev0,policy=0xb0000,cbitpos=51,reduced-phys-bits=1,allowed-sev-features=true,secure-avic=true

Following tests are done:

1) Boot to Prompt using initramfs and ubuntu fs.
2) Verified timer and IPI as part of the guest bootup.
3) Verified long run SCF TORTURE IPI test.

[1] https://github.com/AMDESE/linux-kvm/tree/savic-host-latest
[2] https://github.com/AMDESE/qemu/tree/secure-avic

Change since v1

- Added Kexec support.
- Instead of doing a 2M aligned allocation for backing pages,
allocate individual PAGE_SIZE pages for vCPUs.
- Instead of reading Extended Topology Enumeration CPUID, APIC_ID
value is read from Hv and updated in APIC backing page. Hv returned
ID is checked for any duplicates.
- Propagate all LVT* register reads and writes to Hv.
- Check that Secure AVIC control MSR is not intercepted by Hv.
- Fix EOI handling for level-triggered interrupts.
- Misc cleanups and commit log updates.

Kishon Vijay Abraham I (5):
x86/apic: Support LAPIC timer for Secure AVIC
x86/sev: Initialize VGIF for secondary VCPUs for Secure AVIC
x86/apic: Add support to send NMI IPI for Secure AVIC
x86/sev: Enable NMI support for Secure AVIC
x86/sev: Indicate SEV-SNP guest supports Secure AVIC

Neeraj Upadhyay (12):
x86/apic: Add new driver for Secure AVIC
x86/apic: Initialize Secure AVIC APIC backing page
x86/apic: Populate .read()/.write() callbacks of Secure AVIC driver
x86/apic: Initialize APIC ID for Secure AVIC
x86/apic: Add update_vector callback for Secure AVIC
x86/apic: Add support to send IPI for Secure AVIC
x86/apic: Allow NMI to be injected from hypervisor for Secure AVIC
x86/apic: Read and write LVT* APIC registers from HV for SAVIC guests
x86/apic: Handle EOI writes for SAVIC guests
x86/apic: Add kexec support for Secure AVIC
x86/apic: Enable Secure AVIC in Control MSR
x86/sev: Prevent SECURE_AVIC_CONTROL MSR interception for Secure AVIC
guests

arch/x86/Kconfig | 14 +
arch/x86/boot/compressed/sev.c | 4 +-
arch/x86/coco/core.c | 3 +
arch/x86/coco/sev/core.c | 145 +++++++-
arch/x86/include/asm/apic.h | 4 +
arch/x86/include/asm/apicdef.h | 2 +
arch/x86/include/asm/msr-index.h | 9 +-
arch/x86/include/asm/sev.h | 10 +
arch/x86/include/uapi/asm/svm.h | 3 +
arch/x86/kernel/apic/Makefile | 1 +
arch/x86/kernel/apic/apic.c | 9 +
arch/x86/kernel/apic/vector.c | 8 +
arch/x86/kernel/apic/x2apic_savic.c | 530 ++++++++++++++++++++++++++++
include/linux/cc_platform.h | 8 +
14 files changed, 743 insertions(+), 7 deletions(-)
create mode 100644 arch/x86/kernel/apic/x2apic_savic.c

base-commit: 0f966b1992694763de4dae6bdf817c5c1c6fc66d
--
2.34.1