Re: [PATCH v2 8/8] KVM: x86: Fold decache_cr3() into cache_reg()

[Paolo Bonzini]

On 27/09/19 23:45, Sean Christopherson wrote:
> Handle caching CR3 (from VMX's VMCS) into struct kvm_vcpu via the common
> cache_reg() callback and drop the dedicated decache_cr3().  The name
> decache_cr3() is somewhat confusing as the caching behavior of CR3
> follows that of GPRs, RFLAGS and PDPTRs, (handled via cache_reg()), and
> has nothing in common with the caching behavior of CR0/CR4 (whose
> decache_cr{0,4}_guest_bits() likely provided the 'decache' verbiage).
> 
> Note, this effectively adds a BUG() if KVM attempts to cache CR3 on SVM.
> Opportunistically add a WARN_ON_ONCE() in VMX to provide an equivalent
> check.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

BUG() is a bit heavy, I'll squash this instead:

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index b8885bc0e7d7..e479ea9bc9da 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -2376,7 +2376,7 @@ static void svm_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
 		load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu));
 		break;
 	default:
-		BUG();
+		WARN_ON_ONCE(1);
 	}
 }
 

Since the value that is never cached, literally nothing could go wrong at
least in theory.

Paolo


> ---
>  arch/x86/include/asm/kvm_host.h |  1 -
>  arch/x86/kvm/kvm_cache_regs.h   |  2 +-
>  arch/x86/kvm/svm.c              |  5 -----
>  arch/x86/kvm/vmx/vmx.c          | 15 ++++++---------
>  4 files changed, 7 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index a27f7f6b6b7a..0411dc0a27b0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1040,7 +1040,6 @@ struct kvm_x86_ops {
>  			    struct kvm_segment *var, int seg);
>  	void (*get_cs_db_l_bits)(struct kvm_vcpu *vcpu, int *db, int *l);
>  	void (*decache_cr0_guest_bits)(struct kvm_vcpu *vcpu);
> -	void (*decache_cr3)(struct kvm_vcpu *vcpu);
>  	void (*decache_cr4_guest_bits)(struct kvm_vcpu *vcpu);
>  	void (*set_cr0)(struct kvm_vcpu *vcpu, unsigned long cr0);
>  	void (*set_cr3)(struct kvm_vcpu *vcpu, unsigned long cr3);
> diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
> index 9c2bc528800b..f18177cd0030 100644
> --- a/arch/x86/kvm/kvm_cache_regs.h
> +++ b/arch/x86/kvm/kvm_cache_regs.h
> @@ -145,7 +145,7 @@ static inline ulong kvm_read_cr4_bits(struct kvm_vcpu *vcpu, ulong mask)
>  static inline ulong kvm_read_cr3(struct kvm_vcpu *vcpu)
>  {
>  	if (!kvm_register_is_available(vcpu, VCPU_EXREG_CR3))
> -		kvm_x86_ops->decache_cr3(vcpu);
> +		kvm_x86_ops->cache_reg(vcpu, VCPU_EXREG_CR3);
>  	return vcpu->arch.cr3;
>  }
>  
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index f8ecb6df5106..3102c44c12c6 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2517,10 +2517,6 @@ static void svm_decache_cr0_guest_bits(struct kvm_vcpu *vcpu)
>  {
>  }
>  
> -static void svm_decache_cr3(struct kvm_vcpu *vcpu)
> -{
> -}
> -
>  static void svm_decache_cr4_guest_bits(struct kvm_vcpu *vcpu)
>  {
>  }
> @@ -7208,7 +7204,6 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = {
>  	.get_cpl = svm_get_cpl,
>  	.get_cs_db_l_bits = kvm_get_cs_db_l_bits,
>  	.decache_cr0_guest_bits = svm_decache_cr0_guest_bits,
> -	.decache_cr3 = svm_decache_cr3,
>  	.decache_cr4_guest_bits = svm_decache_cr4_guest_bits,
>  	.set_cr0 = svm_set_cr0,
>  	.set_cr3 = svm_set_cr3,
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ed03d0cd1cc8..c84798026e85 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2188,7 +2188,12 @@ static void vmx_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
>  		if (enable_ept)
>  			ept_save_pdptrs(vcpu);
>  		break;
> +	case VCPU_EXREG_CR3:
> +		if (enable_unrestricted_guest || (enable_ept && is_paging(vcpu)))
> +			vcpu->arch.cr3 = vmcs_readl(GUEST_CR3);
> +		break;
>  	default:
> +		WARN_ON_ONCE(1);
>  		break;
>  	}
>  }
> @@ -2859,13 +2864,6 @@ static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu)
>  	vcpu->arch.cr0 |= vmcs_readl(GUEST_CR0) & cr0_guest_owned_bits;
>  }
>  
> -static void vmx_decache_cr3(struct kvm_vcpu *vcpu)
> -{
> -	if (enable_unrestricted_guest || (enable_ept && is_paging(vcpu)))
> -		vcpu->arch.cr3 = vmcs_readl(GUEST_CR3);
> -	kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
> -}
> -
>  static void vmx_decache_cr4_guest_bits(struct kvm_vcpu *vcpu)
>  {
>  	ulong cr4_guest_owned_bits = vcpu->arch.cr4_guest_owned_bits;
> @@ -2910,7 +2908,7 @@ static void ept_update_paging_mode_cr0(unsigned long *hw_cr0,
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  
>  	if (!kvm_register_is_available(vcpu, VCPU_EXREG_CR3))
> -		vmx_decache_cr3(vcpu);
> +		vmx_cache_reg(vcpu, VCPU_EXREG_CR3);
>  	if (!(cr0 & X86_CR0_PG)) {
>  		/* From paging/starting to nonpaging */
>  		exec_controls_setbit(vmx, CPU_BASED_CR3_LOAD_EXITING |
> @@ -7792,7 +7790,6 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
>  	.get_cpl = vmx_get_cpl,
>  	.get_cs_db_l_bits = vmx_get_cs_db_l_bits,
>  	.decache_cr0_guest_bits = vmx_decache_cr0_guest_bits,
> -	.decache_cr3 = vmx_decache_cr3,
>  	.decache_cr4_guest_bits = vmx_decache_cr4_guest_bits,
>  	.set_cr0 = vmx_set_cr0,
>  	.set_cr3 = vmx_set_cr3,
>