[PATCH v2] KVM: SVM: Inhibit AVIC on SNP-enabled system without HvInUseWrAllowed feature

Suravee Suthikulpanit
2024-10-18T08:52:34Z

On SNP-enabled system, VMRUN marks AVIC Backing Page as in-use while
the guest is running for both secure and non-secure guest. Any hypervisor
write to the in-use vCPU's AVIC backing page (e.g. to inject an interrupt)
will generate unexpected #PF in the host.

Currently, attempt to run AVIC guest would result in the following error:

BUG: unable to handle page fault for address: ff3a442e549cc270
#PF: supervisor write access in kernel mode
#PF: error_code(0x80000003) - RMP violation
PGD b6ee01067 P4D b6ee02067 PUD 10096d063 PMD 11c540063 PTE 80000001149cc163
SEV-SNP: PFN 0x1149cc unassigned, dumping non-zero entries in 2M PFN region: [0x114800 - 0x114a00]
...

Newer AMD system is enhanced to allow hypervisor to modify the backing page
for non-secure guest on SNP-enabled system. This enhancement is available
when the CPUID Fn8000_001F_EAX bit 30 is set (HvInUseWrAllowed).

This table describes AVIC support matrix w.r.t. SNP enablement:

| Non-SNP system | SNP system
-----------------------------------------------------
Non-SNP guest | AVIC Activate | AVIC Activate iff
| | HvInuseWrAllowed=1
-----------------------------------------------------
SNP guest | N/A | Secure AVIC
| | x2APIC only

Introduce APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED to deactivate AVIC
when the feature is not available on SNP-enabled system.

See the AMD64 Architecture Programmer’s Manual (APM) Volume 2 for detail.
(https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/
programmer-references/40332.pdf)

Fixes: 216d106c7ff7 ("x86/sev: Add SEV-SNP host initialization support")
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
---

Change log v2:
* Use APICv inhibit bit instead of disabling AVIC in driver.

arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/kvm_host.h | 9 ++++++++-
arch/x86/kvm/svm/avic.c | 6 ++++++
arch/x86/kvm/svm/svm.h | 3 ++-
4 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index dd4682857c12..921b6de80e24 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -448,6 +448,7 @@
#define X86_FEATURE_SME_COHERENT (19*32+10) /* AMD hardware-enforced cache coherency */
#define X86_FEATURE_DEBUG_SWAP (19*32+14) /* "debug_swap" AMD SEV-ES full debug state swap support */
#define X86_FEATURE_SVSM (19*32+28) /* "svsm" SVSM present */
+#define X86_FEATURE_HV_INUSE_WR_ALLOWED (19*32+30) /* Write to in-use hypervisor-owned pages allowed */

/* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
#define X86_FEATURE_NO_NESTED_DATA_BP (20*32+ 0) /* No Nested Data Breakpoints */
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4a68cb3eba78..1fef50025512 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1276,6 +1276,12 @@ enum kvm_apicv_inhibit {
*/
APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED,

+ /*
+ * Non-SNP guest cannot activate AVIC on SNP-enabled system w/o
+ * CPUID HvInUseWrAllowed feature.
+ */
+ APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED,
+
NR_APICV_INHIBIT_REASONS,
};

@@ -1294,7 +1300,8 @@ enum kvm_apicv_inhibit {
__APICV_INHIBIT_REASON(IRQWIN), \
__APICV_INHIBIT_REASON(PIT_REINJ), \
__APICV_INHIBIT_REASON(SEV), \
- __APICV_INHIBIT_REASON(LOGICAL_ID_ALIASED)
+ __APICV_INHIBIT_REASON(LOGICAL_ID_ALIASED), \
+ __APICV_INHIBIT_REASON(HVINUSEWR_NOT_ALLOWED)

struct kvm_arch {
unsigned long n_used_mmu_pages;
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 4b74ea91f4e6..cc4f0c00334a 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -202,6 +202,12 @@ int avic_vm_init(struct kvm *kvm)
if (!enable_apicv)
return 0;

+ if (cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
+ !boot_cpu_has(X86_FEATURE_HV_INUSE_WR_ALLOWED)) {
+ pr_debug("APICv Inhibit due to Missing HvInUseWrAllowed.\n");
+ kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED);
+ }
+
/* Allocating physical APIC ID table (4KB) */
p_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
if (!p_page)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 76107c7d0595..13046bad2d6e 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -682,7 +682,8 @@ extern struct kvm_x86_nested_ops svm_nested_ops;
BIT(APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED) | \
BIT(APICV_INHIBIT_REASON_APIC_ID_MODIFIED) | \
BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED) | \
- BIT(APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED) \
+ BIT(APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED) | \
+ BIT(APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED) \
)

bool avic_hardware_setup(void);
--
2.34.1