Re: [PATCH 18/18] KVM: selftests: Drop @selector from segment helpers

From: Ackerley Tng <ackerleytng@google.com>
To: Sean Christopherson <seanjc@google.com>,
	Marc Zyngier <maz@kernel.org>,
	 Oliver Upton <oliver.upton@linux.dev>,
	Paolo Bonzini <pbonzini@redhat.com>,
	 Christian Borntraeger <borntraeger@linux.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	 Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Anup Patel <anup@brainfault.org>,
	 Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	 Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	 kvm@vger.kernel.org, kvm-riscv@lists.infradead.org,
	 linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 18/18] KVM: selftests: Drop @selector from segment helpers
Date: Thu, 28 Mar 2024 02:51:15 +0000	[thread overview]
Message-ID: <diqza5mjkr18.fsf@ctop-sg.c.googlers.com> (raw)
In-Reply-To: <20240314232637.2538648-19-seanjc@google.com>

Sean Christopherson <seanjc@google.com> writes:

> Drop the @selector from the kernel code, data, and TSS builders and
> instead hardcode the respective selector in the helper.  Accepting a
> selector but not a base makes the selector useless, e.g. the data helper
> can't create per-vCPU for FS or GS, and so loading GS with KERNEL_DS is
> the only logical choice.
>
> And for code and TSS, there is no known reason to ever want multiple
> segments, e.g. there are zero plans to support 32-bit kernel code (and
> again, that would require more than just the selector).
>
> If KVM selftests ever do add support for per-vCPU segments, it'd arguably
> be more readable to add a dedicated helper for building/setting the
> per-vCPU segment, and move the common data segment code to an inner
> helper.
>
> Lastly, hardcoding the selector reduces the probability of setting the
> wrong selector in the vCPU versus what was created by the VM in the GDT.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  .../selftests/kvm/lib/x86_64/processor.c      | 29 +++++++++----------
>  1 file changed, 14 insertions(+), 15 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> index dab719ee7734..6abd50d6e59d 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> @@ -438,10 +438,10 @@ static void kvm_seg_fill_gdt_64bit(struct kvm_vm *vm, struct kvm_segment *segp)
>  		desc->base3 = segp->base >> 32;
>  }
>  
> -static void kvm_seg_set_kernel_code_64bit(uint16_t selector, struct kvm_segment *segp)
> +static void kvm_seg_set_kernel_code_64bit(struct kvm_segment *segp)
>  {
>  	memset(segp, 0, sizeof(*segp));
> -	segp->selector = selector;
> +	segp->selector = KERNEL_CS;
>  	segp->limit = 0xFFFFFFFFu;
>  	segp->s = 0x1; /* kTypeCodeData */
>  	segp->type = 0x08 | 0x01 | 0x02; /* kFlagCode | kFlagCodeAccessed
> @@ -452,10 +452,10 @@ static void kvm_seg_set_kernel_code_64bit(uint16_t selector, struct kvm_segment
>  	segp->present = 1;
>  }
>  
> -static void kvm_seg_set_kernel_data_64bit(uint16_t selector, struct kvm_segment *segp)
> +static void kvm_seg_set_kernel_data_64bit(struct kvm_segment *segp)
>  {
>  	memset(segp, 0, sizeof(*segp));
> -	segp->selector = selector;
> +	segp->selector = KERNEL_DS;
>  	segp->limit = 0xFFFFFFFFu;
>  	segp->s = 0x1; /* kTypeCodeData */
>  	segp->type = 0x00 | 0x01 | 0x02; /* kFlagData | kFlagDataAccessed
> @@ -480,13 +480,12 @@ vm_paddr_t addr_arch_gva2gpa(struct kvm_vm *vm, vm_vaddr_t gva)
>  	return vm_untag_gpa(vm, PTE_GET_PA(*pte)) | (gva & ~HUGEPAGE_MASK(level));
>  }
>  
> -static void kvm_seg_set_tss_64bit(vm_vaddr_t base, struct kvm_segment *segp,
> -				  int selector)
> +static void kvm_seg_set_tss_64bit(vm_vaddr_t base, struct kvm_segment *segp)
>  {
>  	memset(segp, 0, sizeof(*segp));
>  	segp->base = base;
>  	segp->limit = 0x67;
> -	segp->selector = selector;
> +	segp->selector = KERNEL_TSS;
>  	segp->type = 0xb;
>  	segp->present = 1;
>  }
> @@ -510,11 +509,11 @@ static void vcpu_init_sregs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
>  	sregs.efer |= (EFER_LME | EFER_LMA | EFER_NX);
>  
>  	kvm_seg_set_unusable(&sregs.ldt);
> -	kvm_seg_set_kernel_code_64bit(KERNEL_CS, &sregs.cs);
> -	kvm_seg_set_kernel_data_64bit(KERNEL_DS, &sregs.ds);
> -	kvm_seg_set_kernel_data_64bit(KERNEL_DS, &sregs.es);
> -	kvm_seg_set_kernel_data_64bit(KERNEL_DS, &sregs.gs);
> -	kvm_seg_set_tss_64bit(vm->arch.tss, &sregs.tr, KERNEL_TSS);
> +	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;
>  	vcpu_sregs_set(vcpu, &sregs);
> @@ -588,13 +587,13 @@ static void vm_init_descriptor_tables(struct kvm_vm *vm)
>  
>  	*(vm_vaddr_t *)addr_gva2hva(vm, (vm_vaddr_t)(&exception_handlers)) = vm->handlers;
>  
> -	kvm_seg_set_kernel_code_64bit(KERNEL_CS, &seg);
> +	kvm_seg_set_kernel_code_64bit(&seg);
>  	kvm_seg_fill_gdt_64bit(vm, &seg);
>  
> -	kvm_seg_set_kernel_data_64bit(KERNEL_DS, &seg);
> +	kvm_seg_set_kernel_data_64bit(&seg);
>  	kvm_seg_fill_gdt_64bit(vm, &seg);
>  
> -	kvm_seg_set_tss_64bit(vm->arch.tss, &seg, KERNEL_TSS);
> +	kvm_seg_set_tss_64bit(vm->arch.tss, &seg);
>  	kvm_seg_fill_gdt_64bit(vm, &seg);
>  }
>  
> -- 
> 2.44.0.291.gc1ea87d7ee-goog

Reviewed-by: Ackerley Tng <ackerleytng@google.com>

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