[PATCH v2 3/4] x86/sev: Fix broken SNP support with KVM module built-in
From: Ashish Kalra
Date: Thu Jan 30 2025 - 20:11:56 EST
From: Ashish Kalra <ashish.kalra@xxxxxxx>
This patch fixes issues with enabling SNP host support and effectively
SNP support which is broken with respect to the KVM module being
built-in.
SNP host support is enabled in snp_rmptable_init() which is invoked as a
device_initcall(). Here device_initcall() is used as snp_rmptable_init()
expects AMD IOMMU SNP support to be enabled prior to it and the AMD
IOMMU driver enables SNP support after PCI bus enumeration.
Now, if kvm_amd module is built-in, it gets initialized before SNP host
support is enabled in snp_rmptable_init() :
[ 10.131811] kvm_amd: TSC scaling supported
[ 10.136384] kvm_amd: Nested Virtualization enabled
[ 10.141734] kvm_amd: Nested Paging enabled
[ 10.146304] kvm_amd: LBR virtualization supported
[ 10.151557] kvm_amd: SEV enabled (ASIDs 100 - 509)
[ 10.156905] kvm_amd: SEV-ES enabled (ASIDs 1 - 99)
[ 10.162256] kvm_amd: SEV-SNP enabled (ASIDs 1 - 99)
[ 10.171508] kvm_amd: Virtual VMLOAD VMSAVE supported
[ 10.177052] kvm_amd: Virtual GIF supported
...
...
[ 10.201648] kvm_amd: in svm_enable_virtualization_cpu
And then svm_x86_ops->enable_virtualization_cpu()
(svm_enable_virtualization_cpu) programs MSR_VM_HSAVE_PA as following:
wrmsrl(MSR_VM_HSAVE_PA, sd->save_area_pa);
So VM_HSAVE_PA is non-zero before SNP support is enabled on all CPUs.
snp_rmptable_init() gets invoked after svm_enable_virtualization_cpu()
as following :
...
[ 11.256138] kvm_amd: in svm_enable_virtualization_cpu
...
[ 11.264918] SEV-SNP: in snp_rmptable_init
This triggers a #GP exception in snp_rmptable_init() when snp_enable()
is invoked to set SNP_EN in SYSCFG MSR:
[ 11.294289] unchecked MSR access error: WRMSR to 0xc0010010 (tried to write 0x0000000003fc0000) at rIP: 0xffffffffaf5d5c28 (native_write_msr+0x8/0x30)
...
[ 11.294404] Call Trace:
[ 11.294482] <IRQ>
[ 11.294513] ? show_stack_regs+0x26/0x30
[ 11.294522] ? ex_handler_msr+0x10f/0x180
[ 11.294529] ? search_extable+0x2b/0x40
[ 11.294538] ? fixup_exception+0x2dd/0x340
[ 11.294542] ? exc_general_protection+0x14f/0x440
[ 11.294550] ? asm_exc_general_protection+0x2b/0x30
[ 11.294557] ? __pfx_snp_enable+0x10/0x10
[ 11.294567] ? native_write_msr+0x8/0x30
[ 11.294570] ? __snp_enable+0x5d/0x70
[ 11.294575] snp_enable+0x19/0x20
[ 11.294578] __flush_smp_call_function_queue+0x9c/0x3a0
[ 11.294586] generic_smp_call_function_single_interrupt+0x17/0x20
[ 11.294589] __sysvec_call_function+0x20/0x90
[ 11.294596] sysvec_call_function+0x80/0xb0
[ 11.294601] </IRQ>
[ 11.294603] <TASK>
[ 11.294605] asm_sysvec_call_function+0x1f/0x30
...
[ 11.294631] arch_cpu_idle+0xd/0x20
[ 11.294633] default_idle_call+0x34/0xd0
[ 11.294636] do_idle+0x1f1/0x230
[ 11.294643] ? complete+0x71/0x80
[ 11.294649] cpu_startup_entry+0x30/0x40
[ 11.294652] start_secondary+0x12d/0x160
[ 11.294655] common_startup_64+0x13e/0x141
[ 11.294662] </TASK>
This #GP exception is getting triggered due to the following errata for
AMD family 19h Models 10h-1Fh Processors:
Processor may generate spurious #GP(0) Exception on WRMSR instruction:
Description:
The Processor will generate a spurious #GP(0) Exception on a WRMSR
instruction if the following conditions are all met:
- the target of the WRMSR is a SYSCFG register.
- the write changes the value of SYSCFG.SNPEn from 0 to 1.
- One of the threads that share the physical core has a non-zero
value in the VM_HSAVE_PA MSR.
The document being referred to above:
https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/revision-guides/57095-PUB_1_01.pdf
To summarize, with kvm_amd module being built-in, KVM/SVM initialization
happens before host SNP is enabled and this SVM initialization
sets VM_HSAVE_PA to non-zero, which then triggers a #GP when
SYSCFG.SNPEn is being set and this will subsequently cause
SNP_INIT(_EX) to fail with INVALID_CONFIG error as SYSCFG[SnpEn] is not
set on all CPUs.
This patch fixes the current SNP host enabling code and effectively SNP
which is broken with respect to the KVM module being built-in.
Essentially SNP host enabling code should be invoked before KVM
initialization, which is currently not the case when KVM is built-in.
Now, snp_rmptable_init() will be called early from iommu_snp_enable()
directly and not invoked via device_initcall() which enables SNP host
support before KVM initialization with kvm_amd module built-in.
Fixes: c3b86e61b756 ("x86/cpufeatures: Enable/unmask SEV-SNP CPU feature")
Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
---
arch/x86/include/asm/sev.h | 2 ++
arch/x86/virt/svm/sev.c | 23 +++++++----------------
2 files changed, 9 insertions(+), 16 deletions(-)
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 5d9685f92e5c..1581246491b5 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -531,6 +531,7 @@ static inline void __init snp_secure_tsc_init(void) { }
#ifdef CONFIG_KVM_AMD_SEV
bool snp_probe_rmptable_info(void);
+int snp_rmptable_init(void);
int snp_lookup_rmpentry(u64 pfn, bool *assigned, int *level);
void snp_dump_hva_rmpentry(unsigned long address);
int psmash(u64 pfn);
@@ -541,6 +542,7 @@ void kdump_sev_callback(void);
void snp_fixup_e820_tables(void);
#else
static inline bool snp_probe_rmptable_info(void) { return false; }
+static inline int snp_rmptable_init(void) { return -ENOSYS; }
static inline int snp_lookup_rmpentry(u64 pfn, bool *assigned, int *level) { return -ENODEV; }
static inline void snp_dump_hva_rmpentry(unsigned long address) {}
static inline int psmash(u64 pfn) { return -ENODEV; }
diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 1dcc027ec77e..42e74a5a7d78 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -505,19 +505,19 @@ static bool __init setup_rmptable(void)
* described in the SNP_INIT_EX firmware command description in the SNP
* firmware ABI spec.
*/
-static int __init snp_rmptable_init(void)
+int __init snp_rmptable_init(void)
{
unsigned int i;
u64 val;
- if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
- return 0;
+ if (WARN_ON_ONCE(!cc_platform_has(CC_ATTR_HOST_SEV_SNP)))
+ return -ENOSYS;
- if (!amd_iommu_snp_en)
- goto nosnp;
+ if (WARN_ON_ONCE(!amd_iommu_snp_en))
+ return -ENOSYS;
if (!setup_rmptable())
- goto nosnp;
+ return -ENOSYS;
/*
* Check if SEV-SNP is already enabled, this can happen in case of
@@ -530,7 +530,7 @@ static int __init snp_rmptable_init(void)
/* Zero out the RMP bookkeeping area */
if (!clear_rmptable_bookkeeping()) {
free_rmp_segment_table();
- goto nosnp;
+ return -ENOSYS;
}
/* Zero out the RMP entries */
@@ -562,17 +562,8 @@ static int __init snp_rmptable_init(void)
crash_kexec_post_notifiers = true;
return 0;
-
-nosnp:
- cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
- return -ENOSYS;
}
-/*
- * This must be called after the IOMMU has been initialized.
- */
-device_initcall(snp_rmptable_init);
-
static void set_rmp_segment_info(unsigned int segment_shift)
{
rmp_segment_shift = segment_shift;
--
2.34.1