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

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Message-ID: <b0516341-bce2-41c6-ac43-0d5181bc8af2@intel.com>
Date: Thu, 30 Oct 2025 20:52:48 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Sagi Shahar <sagis@...gle.com>, <linux-kselftest@...r.kernel.org>, "Paolo
 Bonzini" <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, "Sean
 Christopherson" <seanjc@...gle.com>, Ackerley Tng <ackerleytng@...gle.com>,
	Ryan Afranji <afranji@...gle.com>, Andrew Jones <ajones@...tanamicro.com>,
	Isaku Yamahata <isaku.yamahata@...el.com>, Erdem Aktas
	<erdemaktas@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, "Roger
 Wang" <runanwang@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>, "Oliver
 Upton" <oliver.upton@...ux.dev>, "Pratik R. Sampat"
	<pratikrajesh.sampat@....com>, Ira Weiny <ira.weiny@...el.com>, Chao Gao
	<chao.gao@...el.com>, Chenyi Qiang <chenyi.qiang@...el.com>
CC: <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: 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@...ux.intel.com>
> Signed-off-by: Sagi Shahar <sagis@...gle.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