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

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Message-ID: <YhA4WkeqPxbA3IEJ@google.com>
Date:   Sat, 19 Feb 2022 00:22:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 13/18] KVM: x86: reset and reinitialize the MMU in
 __set_sregs_common

On Thu, Feb 17, 2022, Paolo Bonzini wrote:
> Do a full unload of the MMU in KVM_SET_SREGS and KVM_SEST_REGS2, in
> 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>
> ---

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

> +	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;

Eww (not your code, just this whole pile).  It might be worth calling out in the
changelog that calling kvm_init_mmu() before load_pdptrs() will (subtly) _not_
impact the functionality of load_pdptrs().  If the MMU is nested, kvm_init_mmu()
will modify vcpu->arch.nested_mmu, whereas kvm_translate_gpa() will walk
vcpu->arch.guest_mmu.  And if the MMU is not nested, kvm_translate_gpa() will not
consuming vcpu->arch.mmu other than to check if it's == &guest_mmu.

> -		}
>  		srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  	}
>