Re: [PATCH v8 06/30] KVM: selftests: Add helper functions to create TDX VMs

[Edgecombe, Rick P]

On Thu, 2025-08-07 at 13:16 -0700, Sagi Shahar wrote:
> +#define XFEATURE_MASK_CET (XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)
> +
> +static void tdx_apply_cpuid_restrictions(struct kvm_cpuid2 *cpuid_data)
> +{
> +	for (int i = 0; i < cpuid_data->nent; i++) {
> +		struct kvm_cpuid_entry2 *e = &cpuid_data->entries[i];
> +
> +		if (e->function == 0xd && e->index == 0) {
> +			/*
> +			 * TDX module requires both XTILE_{CFG, DATA} to be set.
> +			 * Both bits are required for AMX to be functional.
> +			 */
> +			if ((e->eax & XFEATURE_MASK_XTILE) !=
> +			    XFEATURE_MASK_XTILE) {
> +				e->eax &= ~XFEATURE_MASK_XTILE;
> +			}
> +		}
> +		if (e->function == 0xd && e->index == 1) {
> +			/*
> +			 * TDX doesn't support LBR yet.
> +			 * Disable bits from the XCR0 register.
> +			 */
> +			e->ecx &= ~XFEATURE_MASK_LBR;
> +			/*
> +			 * TDX modules requires both CET_{U, S} to be set even
> +			 * if only one is supported.
> +			 */
> +			if (e->ecx & XFEATURE_MASK_CET)
> +				e->ecx |= XFEATURE_MASK_CET;
> +		}
> +	}
> +}

Since this is only going to be used control the directly configurable bits, do
we really need to do this? SET_CPUID2 will just get what comes out of
KVM_TDX_GET_CPUID, so it should pick up the correct values.


<snip>

> +
> +static void tdx_td_init(struct kvm_vm *vm, uint64_t attributes)
> +{
> +	struct kvm_tdx_init_vm *init_vm;
> +	const struct kvm_cpuid2 *tmp;
> +	struct kvm_cpuid2 *cpuid;
> +
> +	tmp = kvm_get_supported_cpuid();
> +
> +	cpuid = allocate_kvm_cpuid2(KVM_MAX_CPUID_ENTRIES);
> +	memcpy(cpuid, tmp, kvm_cpuid2_size(tmp->nent));
> +	tdx_mask_cpuid_features(cpuid);
> +
> +	init_vm = calloc(1, sizeof(*init_vm) +
> +			 sizeof(init_vm->cpuid.entries[0]) * cpuid->nent);
> +	TEST_ASSERT(init_vm, "vm allocation failed");
> +
> +	memcpy(&init_vm->cpuid, cpuid, kvm_cpuid2_size(cpuid->nent));
> +	free(cpuid);
> +
> +	init_vm->attributes = attributes;
> +
> +	tdx_apply_cpuid_restrictions(&init_vm->cpuid);
> +	tdx_filter_cpuid(vm, &init_vm->cpuid);
> +
> +	tdx_ioctl(vm->fd, KVM_TDX_INIT_VM, 0, init_vm);
> +	free(init_vm);
> +}

We should comment the CPUID twiddling that happens here. It masks, filters, and
applies restrictions. Sounds like all the same thing.