Re: [PATCH v12 04/23] KVM: selftests: Expose function to allocate guest vCPU stack

Hi Sagi,

On 10/28/25 2:20 PM, Sagi Shahar wrote:

TDX guests' registers cannot be initialized directly using

Previous patch used the term "TDX VMs". It will make the changelogs easier to read if the same terms are used consistently.

vcpu_regs_set(), hence the stack pointer needs to be initialized by the guest itself, running boot code beginning at the reset vector.

Sean highlighted in https://lore.kernel.org/lkml/aQN0Qg24tMQ9ckUT@google.com/ that the changelog requirements for selftests should follow Documentation/process/maintainer-kvm-x86.rst. This means that the changelogs should start with a short description of the change followed by the context and problem description (if needed).

Expose the function to allocate the guest stack so that TDX initialization code can allocate it itself and skip the allocation in vm_arch_vcpu_add() in that case.

TDX still allocates the stack in vm_arch_vcpu_add() though, no?

Perhaps something like below (caveat is that KVM style is new to me also so consider this a draft):

Introduce kvm_allocate_vcpu_stack() to allocate a vCPU's stack in preparation for TDX to allocate a vCPU's stack and initialize its stack pointer.

TDX VMs' registers are protected state and cannot be initialized using the KVM_SET_REGS ioctl() that is used for normal VMs. A TDX vCPU's stack address will be a property of the TDX specific boot code that initializes the vCPUs' stack pointers at boot.

Reviewed-by: Binbin Wu binbin.wu@linux.intel.com Signed-off-by: Sagi Shahar sagis@google.com

---

.../selftests/kvm/include/x86/processor.h | 2 ++ tools/testing/selftests/kvm/lib/x86/processor.c | 16 +++++++++++----- 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/x86/processor.h b/tools/testing/selftests/kvm/include/x86/processor.h index 9caeb3de7df6..dba2b3d558d1 100644 --- a/tools/testing/selftests/kvm/include/x86/processor.h +++ b/tools/testing/selftests/kvm/include/x86/processor.h @@ -1120,6 +1120,8 @@ static inline void vcpu_clear_cpuid_feature(struct kvm_vcpu *vcpu, vcpu_set_or_clear_cpuid_feature(vcpu, feature, false); } +vm_vaddr_t kvm_allocate_vcpu_stack(struct kvm_vm *vm);

uint64_t vcpu_get_msr(struct kvm_vcpu *vcpu, uint64_t msr_index); int _vcpu_set_msr(struct kvm_vcpu *vcpu, uint64_t msr_index, uint64_t msr_value); diff --git a/tools/testing/selftests/kvm/lib/x86/processor.c b/tools/testing/selftests/kvm/lib/x86/processor.c index 2d1544e8af6c..2898fe4f6de4 100644 --- a/tools/testing/selftests/kvm/lib/x86/processor.c +++ b/tools/testing/selftests/kvm/lib/x86/processor.c @@ -693,12 +693,9 @@ void vcpu_arch_set_entry_point(struct kvm_vcpu *vcpu, void *guest_code) vcpu_regs_set(vcpu, &regs); } -struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id) +vm_vaddr_t kvm_allocate_vcpu_stack(struct kvm_vm *vm) {

• struct kvm_mp_state mp_state;
• struct kvm_regs regs; vm_vaddr_t stack_vaddr;
• struct kvm_vcpu *vcpu;

stack_vaddr = __vm_vaddr_alloc(vm, DEFAULT_STACK_PGS * getpagesize(), DEFAULT_GUEST_STACK_VADDR_MIN, @@ -719,6 +716,15 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id) "__vm_vaddr_alloc() did not provide a page-aligned address"); stack_vaddr -= 8;

• return stack_vaddr;

+}

+struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id) +{

• struct kvm_mp_state mp_state;
• struct kvm_regs regs;
• struct kvm_vcpu *vcpu;

Even though the original code did not do so I'd propose these declarations be in reverse fir order.

• vcpu = __vm_vcpu_add(vm, vcpu_id); vcpu_init_cpuid(vcpu, kvm_get_supported_cpuid()); vcpu_init_sregs(vm, vcpu);

@@ -727,7 +733,7 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id) /* Setup guest general purpose registers */ vcpu_regs_get(vcpu, &regs); regs.rflags = regs.rflags | 0x2;

• regs.rsp = stack_vaddr;

• regs.rsp = kvm_allocate_vcpu_stack(vm); vcpu_regs_set(vcpu, &regs);

/* Setup the MP state */

Reinette