linux-kernel - Re: [PATCH v2 13/18] KVM: x86: reset and reinitialize the MMU in __set_sregs

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Date:   Wed, 23 Feb 2022 18:48:33 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: Re: [PATCH v2 13/18] KVM: x86: reset and reinitialize the MMU in
 __set_sregs_common

On Thu, 2022-02-17 at 16:03 -0500, Paolo Bonzini wrote:
> Do a full unload of the MMU in KVM_SET_SREGS and KVM_SEST_REGS2, in
Typo
> preparation for not doing so in kvm_mmu_reset_context.  There is no
> need to delay the reset until after the return, so do it directly in
> the __set_sregs_common function and remove the mmu_reset_needed output
> parameter.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 32 +++++++++++++-------------------
>  1 file changed, 13 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6aefd7ac7039..f10878aa5b20 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10730,7 +10730,7 @@ static bool kvm_is_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  }
>  
>  static int __set_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs,
> -		int *mmu_reset_needed, bool update_pdptrs)
> +			      int update_pdptrs)
>  {
>  	struct msr_data apic_base_msr;
>  	int idx;
> @@ -10755,29 +10755,31 @@ static int __set_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs,
>  	static_call(kvm_x86_set_gdt)(vcpu, &dt);
>  
>  	vcpu->arch.cr2 = sregs->cr2;
> -	*mmu_reset_needed |= kvm_read_cr3(vcpu) != sregs->cr3;
> +
> +	if (vcpu->arch.efer != sregs->efer ||
> +	    kvm_read_cr0(vcpu) != sregs->cr0 ||
> +	    vcpu->arch.cr3 != sregs->cr3 || !update_pdptrs ||
> +	    kvm_read_cr4(vcpu) != sregs->cr4)
> +		kvm_mmu_unload(vcpu);

Should it be (update_pdptrs && is_pae_paging(vcpu)) instead?

> +
>  	vcpu->arch.cr3 = sregs->cr3;
>  	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
>  	static_call_cond(kvm_x86_post_set_cr3)(vcpu, sregs->cr3);
>  
>  	kvm_set_cr8(vcpu, sregs->cr8);
>  
> -	*mmu_reset_needed |= vcpu->arch.efer != sregs->efer;
>  	static_call(kvm_x86_set_efer)(vcpu, sregs->efer);
>  
> -	*mmu_reset_needed |= kvm_read_cr0(vcpu) != sregs->cr0;
>  	static_call(kvm_x86_set_cr0)(vcpu, sregs->cr0);
>  	vcpu->arch.cr0 = sregs->cr0;
>  
> -	*mmu_reset_needed |= kvm_read_cr4(vcpu) != sregs->cr4;
>  	static_call(kvm_x86_set_cr4)(vcpu, sregs->cr4);
>  
> +	kvm_init_mmu(vcpu);
>  	if (update_pdptrs) {
>  		idx = srcu_read_lock(&vcpu->kvm->srcu);
> -		if (is_pae_paging(vcpu)) {
> +		if (is_pae_paging(vcpu))
>  			load_pdptrs(vcpu, kvm_read_cr3(vcpu));
> -			*mmu_reset_needed = 1;
> -		}
>  		srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  	}
>  
> @@ -10805,15 +10807,11 @@ static int __set_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs,
>  static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  {
>  	int pending_vec, max_bits;
> -	int mmu_reset_needed = 0;
> -	int ret = __set_sregs_common(vcpu, sregs, &mmu_reset_needed, true);
> +	int ret = __set_sregs_common(vcpu, sregs, true);
>  
>  	if (ret)
>  		return ret;
>  
> -	if (mmu_reset_needed)
> -		kvm_mmu_reset_context(vcpu);
> -
>  	max_bits = KVM_NR_INTERRUPTS;
>  	pending_vec = find_first_bit(
>  		(const unsigned long *)sregs->interrupt_bitmap, max_bits);
> @@ -10828,7 +10826,6 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  
>  static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2)
>  {
> -	int mmu_reset_needed = 0;
>  	bool valid_pdptrs = sregs2->flags & KVM_SREGS2_FLAGS_PDPTRS_VALID;
>  	bool pae = (sregs2->cr0 & X86_CR0_PG) && (sregs2->cr4 & X86_CR4_PAE) &&
>  		!(sregs2->efer & EFER_LMA);
> @@ -10840,8 +10837,7 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2)
>  	if (valid_pdptrs && (!pae || vcpu->arch.guest_state_protected))
>  		return -EINVAL;
>  
> -	ret = __set_sregs_common(vcpu, (struct kvm_sregs *)sregs2,
> -				 &mmu_reset_needed, !valid_pdptrs);
> +	ret = __set_sregs_common(vcpu, (struct kvm_sregs *)sregs2, !valid_pdptrs);
>  	if (ret)
>  		return ret;
>  
> @@ -10850,11 +10846,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2)
>  			kvm_pdptr_write(vcpu, i, sregs2->pdptrs[i]);
>  
>  		kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
> -		mmu_reset_needed = 1;
>  		vcpu->arch.pdptrs_from_userspace = true;
> +		/* kvm_mmu_reload will be called on the next entry.  */
Could you elaborate on this? 

In theory if set_sregs2 only changed the pdptrs, without changing anything else
which won't really happen in practice but can in theory, then there will
be no mmu reset with new code IMHO.

Best regards,
	Maxim Levitsky


>  	}
> -	if (mmu_reset_needed)
> -		kvm_mmu_reset_context(vcpu);
>  	return 0;
>  }
>