Re: [PATCH v3 6/7] KVM: SVM: Add support to initialize SEV/SNP functionality in KVM

Kalra, Ashish
2025-01-13T15:03:29Z

On 1/10/2025 6:40 PM, Sean Christopherson wrote:
> On Fri, Jan 10, 2025, Ashish Kalra wrote:
>> It looks like i have hit a serious blocker issue with this approach of moving
>> SEV/SNP initialization to KVM module load time.
>>
>> While testing with kvm_amd and PSP driver built-in, it looks like kvm_amd
>> driver is being loaded/initialized before PSP driver is loaded, and that
>> causes sev_platform_init() call from sev_hardware_setup(kvm_amd) to fail:
>>
>> [ 10.717898] kvm_amd: TSC scaling supported
>> [ 10.722470] kvm_amd: Nested Virtualization enabled
>> [ 10.727816] kvm_amd: Nested Paging enabled
>> [ 10.732388] kvm_amd: LBR virtualization supported
>> [ 10.737639] kvm_amd: SEV enabled (ASIDs 100 - 509)
>> [ 10.742985] kvm_amd: SEV-ES enabled (ASIDs 1 - 99)
>> [ 10.748333] kvm_amd: SEV-SNP enabled (ASIDs 1 - 99)
>> [ 10.753768] PSP driver not init <<<---- sev_platform_init() returns failure as PSP driver is still not initialized
>> [ 10.757563] kvm_amd: Virtual VMLOAD VMSAVE supported
>> [ 10.763124] kvm_amd: Virtual GIF supported
>> ...
>> ...
>> [ 12.514857] ccp 0000:23:00.1: enabling device (0000 -> 0002)
>> [ 12.521691] ccp 0000:23:00.1: no command queues available
>> [ 12.527991] ccp 0000:23:00.1: sev enabled
>> [ 12.532592] ccp 0000:23:00.1: psp enabled
>> [ 12.537382] ccp 0000:a2:00.1: enabling device (0000 -> 0002)
>> [ 12.544389] ccp 0000:a2:00.1: no command queues available
>> [ 12.550627] ccp 0000:a2:00.1: psp enabled
>>
>> depmod -> modules.builtin show kernel/arch/x86/kvm/kvm_amd.ko higher on the list and before kernel/drivers/crypto/ccp/ccp.ko
>>
>> modules.builtin:
>> kernel/arch/x86/kvm/kvm.ko
>> kernel/arch/x86/kvm/kvm-amd.ko
>> ...
>> ...
>> kernel/drivers/crypto/ccp/ccp.ko
>>
>> I believe that the modules which are compiled first get called first and it
>> looks like that the only way to change the order for builtin modules is by
>> changing which makefiles get compiled first ?
>>
>> Is there a way to change the load order of built-in modules and/or change
>> dependency of built-in modules ?
>
> The least awful option I know of would be to have the PSP use a higher priority
> initcall type so that it runs before the standard initcalls. When compiled as
> a module, all initcall types are #defined to module_init.
>
> E.g. this should work, /cross fingers
>
> diff --git a/drivers/crypto/ccp/sp-dev.c b/drivers/crypto/ccp/sp-dev.c
> index 7eb3e4668286..02c49fbf6198 100644
> --- a/drivers/crypto/ccp/sp-dev.c
> +++ b/drivers/crypto/ccp/sp-dev.c
> @@ -295,5 +295,6 @@ static void __exit sp_mod_exit(void)
> #endif
> }
>
> -module_init(sp_mod_init);
> +/* The PSP needs to be initialized before dependent modules, e.g. before KVM. */
> +subsys_initcall(sp_mod_init);
> module_exit(sp_mod_exit);

Thanks for the suggestion, but there are actually two major issues here:

With the above change, PSP driver initialization fails as following:

...
[ 7.274005] pci 0000:20:08.1: bridge window [mem 0xf6200000-0xf64fffff]: not claimed; can't enable device
[ 7.277945] pci 0000:20:08.1: Error enabling bridge (-22), continuing
[ 7.281947] ccp 0000:23:00.1: BAR 2 [mem 0xf6300000-0xf63fffff]: not claimed; can't enable device
[ 7.285945] ccp 0000:23:00.1: pcim_enable_device failed (-22)
[ 7.289943] ccp 0000:23:00.1: initialization failed
[ 7.293944] ccp 0000:23:00.1: probe with driver ccp failed with error -22
[ 7.301981] pci 0000:a0:08.1: bridge window [mem 0xb6200000-0xb63fffff]: not claimed; can't enable device
[ 7.313956] pci 0000:a0:08.1: Error enabling bridge (-22), continuing
[ 7.321947] ccp 0000:a2:00.1: BAR 2 [mem 0xb6200000-0xb62fffff]: not claimed; can't enable device
[ 7.329945] ccp 0000:a2:00.1: pcim_enable_device failed (-22)
[ 7.337943] ccp 0000:a2:00.1: initialization failed
[ 7.341946] ccp 0000:a2:00.1: probe with driver ccp failed with error -22
...

It looks as PCI bus resource allocation is still not done, hence PSP driver cannot be enabled as early as subsys_initcall,
it can be initialized probably via device_initcall(), but then that will be too late as kvm_amd would have been initialized before that.

Additionally, it looks like that there is an issue with SNP host support being enabled with kvm_amd module being built-in:

SNP host support is enabled in snp_rmptable_init() in arch/x86/virt/svm/sev.c, 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 is initialized 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)
[ 10.167701] PSP driver not init
[ 10.171508] kvm_amd: Virtual VMLOAD VMSAVE supported
[ 10.177052] kvm_amd: Virtual GIF supported
...
...
[ 10.201648] kvm_amd: in svm_enable_virtualization_cpu WRMSR VM_HSAVE_PA non-zero

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 WRMSR VM_HSAVE_PA non-zero
...
[ 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 suggested workaround is when enabling SNP, program VM_HSAVE_PA to 0h on both threads that share a physical core before setting SYSCFG.SNPEn

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

Therefore, 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 this #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.

So it looks like the current SNP host enabling code and effectively SNP 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.

Additionally, the PSP driver probably needs to be initialized at device_initcall level if it is built-in, but that is much later than KVM
module initialization, therefore, that is blocker for moving SEV/SNP initialization to KVM module load time instead of PSP module probe time.
Do note that i have verified and tested that PSP module initialization works when invoked as a device_initcall().

Thanks,
Ashish