Re: [PATCH v8 02/30] KVM: selftests: Expose function that sets up sregs based on VM's mode

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 07, 2025, Sagi Shahar wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 

Make changelogs standalone, i.e. don't rely on the subject/shortlog for context.

> This allows initializing sregs without setting vCPU registers in
> KVM.
> 
> No functional change intended.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Sagi Shahar <sagis@google.com>
> ---
>  .../selftests/kvm/include/x86/processor.h     |  1 +
>  .../testing/selftests/kvm/lib/x86/processor.c | 45 ++++++++++---------
>  2 files changed, 25 insertions(+), 21 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/include/x86/processor.h b/tools/testing/selftests/kvm/include/x86/processor.h
> index b11b5a53ebd5..f2eb764cbd7c 100644
> --- a/tools/testing/selftests/kvm/include/x86/processor.h
> +++ b/tools/testing/selftests/kvm/include/x86/processor.h
> @@ -1025,6 +1025,7 @@ static inline struct kvm_cpuid2 *allocate_kvm_cpuid2(int nr_entries)
>  }
>  
>  void vcpu_init_cpuid(struct kvm_vcpu *vcpu, const struct kvm_cpuid2 *cpuid);
> +void vcpu_setup_mode_sregs(struct kvm_vm *vm, struct kvm_sregs *sregs);
>  
>  static inline void vcpu_get_cpuid(struct kvm_vcpu *vcpu)
>  {
> diff --git a/tools/testing/selftests/kvm/lib/x86/processor.c b/tools/testing/selftests/kvm/lib/x86/processor.c
> index a92dc1dad085..002303e2a572 100644
> --- a/tools/testing/selftests/kvm/lib/x86/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86/processor.c
> @@ -488,34 +488,37 @@ static void kvm_seg_set_tss_64bit(vm_vaddr_t base, struct kvm_segment *segp)
>  	segp->present = 1;
>  }
>  
> -static void vcpu_init_sregs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
> +void vcpu_setup_mode_sregs(struct kvm_vm *vm, struct kvm_sregs *sregs)
>  {
> -	struct kvm_sregs sregs;
> -
>  	TEST_ASSERT_EQ(vm->mode, VM_MODE_PXXV48_4K);
>  
> -	/* Set mode specific system register values. */
> -	vcpu_sregs_get(vcpu, &sregs);
> -
> -	sregs.idt.base = vm->arch.idt;
> -	sregs.idt.limit = NUM_INTERRUPTS * sizeof(struct idt_entry) - 1;
> -	sregs.gdt.base = vm->arch.gdt;
> -	sregs.gdt.limit = getpagesize() - 1;
> +	sregs->idt.base = vm->arch.idt;
> +	sregs->idt.limit = NUM_INTERRUPTS * sizeof(struct idt_entry) - 1;
> +	sregs->gdt.base = vm->arch.gdt;
> +	sregs->gdt.limit = getpagesize() - 1;
>  
> -	sregs.cr0 = X86_CR0_PE | X86_CR0_NE | X86_CR0_PG;
> -	sregs.cr4 |= X86_CR4_PAE | X86_CR4_OSFXSR;
> +	sregs->cr0 = X86_CR0_PE | X86_CR0_NE | X86_CR0_PG;
> +	sregs->cr4 |= X86_CR4_PAE | X86_CR4_OSFXSR;
>  	if (kvm_cpu_has(X86_FEATURE_XSAVE))
> -		sregs.cr4 |= X86_CR4_OSXSAVE;
> -	sregs.efer |= (EFER_LME | EFER_LMA | EFER_NX);
> +		sregs->cr4 |= X86_CR4_OSXSAVE;
> +	sregs->efer |= (EFER_LME | EFER_LMA | EFER_NX);
> +
> +	kvm_seg_set_unusable(&sregs->ldt);
> +	kvm_seg_set_kernel_code_64bit(&sregs->cs);
> +	kvm_seg_set_kernel_data_64bit(&sregs->ds);
> +	kvm_seg_set_kernel_data_64bit(&sregs->es);
> +	kvm_seg_set_kernel_data_64bit(&sregs->gs);
> +	kvm_seg_set_tss_64bit(vm->arch.tss, &sregs->tr);
>  
> -	kvm_seg_set_unusable(&sregs.ldt);
> -	kvm_seg_set_kernel_code_64bit(&sregs.cs);
> -	kvm_seg_set_kernel_data_64bit(&sregs.ds);
> -	kvm_seg_set_kernel_data_64bit(&sregs.es);
> -	kvm_seg_set_kernel_data_64bit(&sregs.gs);
> -	kvm_seg_set_tss_64bit(vm->arch.tss, &sregs.tr);
> +	sregs->cr3 = vm->pgd;

Add helpers/macros for the few things that are open coded here so that the TDX
code can get the "default" values.  Bouncing data through kvm_sregs is unnecessary
(unless you're trying to win the Obfuscated C contest) and makes it much harder to
understand what TDX actually needs, and why.

IDT.base, GDT.base, and CR3 (vm->pgd) are already available, so something like:

diff --git a/tools/testing/selftests/kvm/lib/x86/processor.c b/tools/testing/selftests/kvm/lib/x86/processor.c
index d4c19ac885a9..83efcf48faad 100644
--- a/tools/testing/selftests/kvm/lib/x86/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86/processor.c
@@ -498,15 +498,13 @@ static void vcpu_init_sregs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
        vcpu_sregs_get(vcpu, &sregs);
 
        sregs.idt.base = vm->arch.idt;
-       sregs.idt.limit = NUM_INTERRUPTS * sizeof(struct idt_entry) - 1;
+       sregs.idt.limit = kvm_get_default_idt_limit();
        sregs.gdt.base = vm->arch.gdt;
-       sregs.gdt.limit = getpagesize() - 1;
+       sregs.gdt.limit = kvm_get_default_gdt_limit();
 
-       sregs.cr0 = X86_CR0_PE | X86_CR0_NE | X86_CR0_PG;
-       sregs.cr4 |= X86_CR4_PAE | X86_CR4_OSFXSR;
-       if (kvm_cpu_has(X86_FEATURE_XSAVE))
-               sregs.cr4 |= X86_CR4_OSXSAVE;
-       sregs.efer |= (EFER_LME | EFER_LMA | EFER_NX);
+       sregs.cr0 = kvm_get_default_cr0();
+       sregs.cr4 |= kvm_get_default_cr4();
+       sregs.efer |= kvm_get_default_efer();
 
        kvm_seg_set_unusable(&sregs.ldt);
        kvm_seg_set_kernel_code_64bit(&sregs.cs);


> +}
> +
> +static void vcpu_init_sregs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_sregs sregs;
>  
> -	sregs.cr3 = vm->pgd;
> +	vcpu_sregs_get(vcpu, &sregs);
> +	vcpu_setup_mode_sregs(vm, &sregs);
>  	vcpu_sregs_set(vcpu, &sregs);
>  }
>  
> -- 
> 2.51.0.rc0.155.g4a0f42376b-goog
>