Re: [PATCH 21/25] KVM: x86: Introduce KVM_TDX_GET_CPUID

Xiaoyao Li
2024-09-30T06:27:06Z

On 8/13/2024 6:48 AM, Rick Edgecombe wrote:
> From: Xiaoyao Li <xiaoyao.li@intel.com>
>
> Implement an IOCTL to allow userspace to read the CPUID bit values for a
> configured TD.
>
> The TDX module doesn't provide the ability to set all CPUID bits. Instead
> some are configured indirectly, or have fixed values. But it does allow
> for the final resulting CPUID bits to be read. This information will be
> useful for userspace to understand the configuration of the TD, and set
> KVM's copy via KVM_SET_CPUID2.
>
> To prevent userspace from starting to use features that might not have KVM
> support yet, filter the reported values by KVM's support CPUID bits.

This patch lacks the documentation of KVM_TDX_GET_CPUID to describe what
it returns and how the returned data is used or expected to be used by
userspace.

Set aside the filtering of KVM's support CPUID bits, KVM_TDX_GET_CPUID
only returns the CPUID leaves that can be readable by TDX module (in
tdx_mask_cpuid()). So what about the leaves that aren't readable? e.g.,
CPUID leaf 6,9,0xc,etc, and leaf 0x40000000 and 0x40000001.

Should userspace to intercept it as the leaves that aren't covered by
KVM_TDX_GET_CPUID are totally controlled by userspace itself and it's
userspace's responsibility to set a sane and valid value?

> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
> uAPI breakout v1:
> - New patch
> ---
> arch/x86/include/uapi/asm/kvm.h | 1 +
> arch/x86/kvm/vmx/tdx.c | 131 ++++++++++++++++++++++++++++++++
> arch/x86/kvm/vmx/tdx.h | 5 ++
> arch/x86/kvm/vmx/tdx_arch.h | 5 ++
> arch/x86/kvm/vmx/tdx_errno.h | 1 +
> 5 files changed, 143 insertions(+)
>
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index b4f12997052d..39636be5c891 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -931,6 +931,7 @@ enum kvm_tdx_cmd_id {
> KVM_TDX_CAPABILITIES = 0,
> KVM_TDX_INIT_VM,
> KVM_TDX_INIT_VCPU,
> + KVM_TDX_GET_CPUID,
>
> KVM_TDX_CMD_NR_MAX,
> };
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index b2ed031ac0d6..fe2bbc2ced41 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -813,6 +813,76 @@ static int __tdx_td_init(struct kvm *kvm, struct td_params *td_params,
> return ret;
> }
>
> +static u64 tdx_td_metadata_field_read(struct kvm_tdx *tdx, u64 field_id,
> + u64 *data)
> +{
> + u64 err;
> +
> + err = tdh_mng_rd(tdx, field_id, data);
> +
> + return err;
> +}
> +
> +#define TDX_MD_UNREADABLE_LEAF_MASK GENMASK(30, 7)
> +#define TDX_MD_UNREADABLE_SUBLEAF_MASK GENMASK(31, 7)
> +
> +static int tdx_mask_cpuid(struct kvm_tdx *tdx, struct kvm_cpuid_entry2 *entry)
> +{
> + u64 field_id = TD_MD_FIELD_ID_CPUID_VALUES;
> + u64 ebx_eax, edx_ecx;
> + u64 err = 0;
> +
> + if (entry->function & TDX_MD_UNREADABLE_LEAF_MASK ||
> + entry->index & TDX_MD_UNREADABLE_SUBLEAF_MASK)
> + return -EINVAL;
> +
> + /*
> + * bit 23:17, REVSERVED: reserved, must be 0;
> + * bit 16, LEAF_31: leaf number bit 31;
> + * bit 15:9, LEAF_6_0: leaf number bits 6:0, leaf bits 30:7 are
> + * implicitly 0;
> + * bit 8, SUBLEAF_NA: sub-leaf not applicable flag;
> + * bit 7:1, SUBLEAF_6_0: sub-leaf number bits 6:0. If SUBLEAF_NA is 1,
> + * the SUBLEAF_6_0 is all-1.
> + * sub-leaf bits 31:7 are implicitly 0;
> + * bit 0, ELEMENT_I: Element index within field;
> + */
> + field_id |= ((entry->function & 0x80000000) ? 1 : 0) << 16;
> + field_id |= (entry->function & 0x7f) << 9;
> + if (entry->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX)
> + field_id |= (entry->index & 0x7f) << 1;
> + else
> + field_id |= 0x1fe;
> +
> + err = tdx_td_metadata_field_read(tdx, field_id, &ebx_eax);
> + if (err) //TODO check for specific errors
> + goto err_out;
> +
> + entry->eax &= (u32) ebx_eax;
> + entry->ebx &= (u32) (ebx_eax >> 32);
> +
> + field_id++;
> + err = tdx_td_metadata_field_read(tdx, field_id, &edx_ecx);
> + /*
> + * It's weird that reading edx_ecx fails while reading ebx_eax
> + * succeeded.
> + */
> + if (WARN_ON_ONCE(err))
> + goto err_out;
> +
> + entry->ecx &= (u32) edx_ecx;
> + entry->edx &= (u32) (edx_ecx >> 32);
> + return 0;
> +
> +err_out:
> + entry->eax = 0;
> + entry->ebx = 0;
> + entry->ecx = 0;
> + entry->edx = 0;
> +
> + return -EIO;
> +}
> +
> static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
> {
> struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> @@ -1038,6 +1108,64 @@ static int __maybe_unused tdx_get_kvm_supported_cpuid(struct kvm_cpuid2 **cpuid)
> return r;
> }
>
> +static int tdx_vcpu_get_cpuid(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
> +{
> + struct kvm_cpuid2 __user *output, *td_cpuid;
> + struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> + struct kvm_cpuid2 *supported_cpuid;
> + int r = 0, i, j = 0;
> +
> + output = u64_to_user_ptr(cmd->data);
> + td_cpuid = kzalloc(sizeof(*td_cpuid) +
> + sizeof(output->entries[0]) * KVM_MAX_CPUID_ENTRIES,
> + GFP_KERNEL);
> + if (!td_cpuid)
> + return -ENOMEM;
> +
> + r = tdx_get_kvm_supported_cpuid(&supported_cpuid);
> + if (r)
> + goto out;
> +
> + for (i = 0; i < supported_cpuid->nent; i++) {
> + struct kvm_cpuid_entry2 *supported = &supported_cpuid->entries[i];
> + struct kvm_cpuid_entry2 *output_e = &td_cpuid->entries[j];
> +
> + *output_e = *supported;
> +
> + /* Only allow values of bits that KVM's supports to be exposed */
> + if (tdx_mask_cpuid(kvm_tdx, output_e))
> + continue;
> +
> + /*
> + * Work around missing support on old TDX modules, fetch
> + * guest maxpa from gfn_direct_bits.
> + */
> + if (output_e->function == 0x80000008) {
> + gpa_t gpa_bits = gfn_to_gpa(kvm_gfn_direct_bits(vcpu->kvm));
> + unsigned int g_maxpa = __ffs(gpa_bits) + 1;
> +
> + output_e->eax &= ~0x00ff0000;
> + output_e->eax |= g_maxpa << 16;
> + }
> +
> + j++;
> + }
> + td_cpuid->nent = j;
> +
> + if (copy_to_user(output, td_cpuid, sizeof(*output))) {
> + r = -EFAULT;
> + goto out;
> + }
> + if (copy_to_user(output->entries, td_cpuid->entries,
> + td_cpuid->nent * sizeof(struct kvm_cpuid_entry2)))
> + r = -EFAULT;
> +
> +out:
> + kfree(td_cpuid);
> + kfree(supported_cpuid);
> + return r;
> +}
> +
> static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
> {
> struct msr_data apic_base_msr;
> @@ -1089,6 +1217,9 @@ int tdx_vcpu_ioctl(struct kvm_vcpu *vcpu, void __user *argp)
> case KVM_TDX_INIT_VCPU:
> ret = tdx_vcpu_init(vcpu, &cmd);
> break;
> + case KVM_TDX_GET_CPUID:
> + ret = tdx_vcpu_get_cpuid(vcpu, &cmd);
> + break;
> default:
> ret = -EINVAL;
> break;
> diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
> index 8349b542836e..7eeb54fbcae1 100644
> --- a/arch/x86/kvm/vmx/tdx.h
> +++ b/arch/x86/kvm/vmx/tdx.h
> @@ -25,6 +25,11 @@ struct kvm_tdx {
> bool finalized;
>
> u64 tsc_offset;
> +
> + /* For KVM_MAP_MEMORY and KVM_TDX_INIT_MEM_REGION. */
> + atomic64_t nr_premapped;
> +
> + struct kvm_cpuid2 *cpuid;
> };
>
> struct vcpu_tdx {
> diff --git a/arch/x86/kvm/vmx/tdx_arch.h b/arch/x86/kvm/vmx/tdx_arch.h
> index d2d7f9cab740..815e74408a34 100644
> --- a/arch/x86/kvm/vmx/tdx_arch.h
> +++ b/arch/x86/kvm/vmx/tdx_arch.h
> @@ -157,4 +157,9 @@ struct td_params {
>
> #define MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM BIT_ULL(20)
>
> +/*
> + * TD scope metadata field ID.
> + */
> +#define TD_MD_FIELD_ID_CPUID_VALUES 0x9410000300000000ULL
> +
> #endif /* __KVM_X86_TDX_ARCH_H */
> diff --git a/arch/x86/kvm/vmx/tdx_errno.h b/arch/x86/kvm/vmx/tdx_errno.h
> index dc3fa2a58c2c..f9dbb3a065cc 100644
> --- a/arch/x86/kvm/vmx/tdx_errno.h
> +++ b/arch/x86/kvm/vmx/tdx_errno.h
> @@ -23,6 +23,7 @@
> #define TDX_FLUSHVP_NOT_DONE 0x8000082400000000ULL
> #define TDX_EPT_WALK_FAILED 0xC0000B0000000000ULL
> #define TDX_EPT_ENTRY_STATE_INCORRECT 0xC0000B0D00000000ULL
> +#define TDX_METADATA_FIELD_NOT_READABLE 0xC0000C0200000000ULL
>
> /*
> * TDX module operand ID, appears in 31:0 part of error code as