linux-kernel - Re: [PATCH 3/3] KVM: x86/mmu: don't skip mmu initialization when mmu root level changes

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Message-ID: <87r1bom5h3.fsf@vitty.brq.redhat.com>
Date:   Wed, 10 Nov 2021 15:48:40 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Wanpeng Li <wanpengli@...cent.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 3/3] KVM: x86/mmu: don't skip mmu initialization when
 mmu root level changes

Maxim Levitsky <mlevitsk@...hat.com> writes:

> When running mix of 32 and 64 bit guests, it is possible to have mmu
> reset with same mmu role but different root level (32 bit vs 64 bit paging)
>
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 354d2ca92df4d..763867475860f 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4745,7 +4745,10 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
>  	union kvm_mmu_role new_role =
>  		kvm_calc_tdp_mmu_root_page_role(vcpu, &regs, false);
>  
> -	if (new_role.as_u64 == context->mmu_role.as_u64)
> +	u8 new_root_level = role_regs_to_root_level(&regs);
> +
> +	if (new_role.as_u64 == context->mmu_role.as_u64 &&
> +	    context->root_level == new_root_level)
>  		return;

role_regs_to_root_level() uses 3 things: CR0.PG, EFER.LMA and CR4.PAE
and two of these three are already encoded into extended mmu role
(kvm_calc_mmu_role_ext()). Could we achieve the same result by adding
EFER.LMA there?

>  
>  	context->mmu_role.as_u64 = new_role.as_u64;
> @@ -4757,7 +4760,7 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
>  	context->get_guest_pgd = get_cr3;
>  	context->get_pdptr = kvm_pdptr_read;
>  	context->inject_page_fault = kvm_inject_page_fault;
> -	context->root_level = role_regs_to_root_level(&regs);
> +	context->root_level = new_root_level;
>  
>  	if (!is_cr0_pg(context))
>  		context->gva_to_gpa = nonpaging_gva_to_gpa;
> @@ -4806,7 +4809,10 @@ static void shadow_mmu_init_context(struct kvm_vcpu *vcpu, struct kvm_mmu *conte
>  				    struct kvm_mmu_role_regs *regs,
>  				    union kvm_mmu_role new_role)
>  {
> -	if (new_role.as_u64 == context->mmu_role.as_u64)
> +	u8 new_root_level = role_regs_to_root_level(regs);
> +
> +	if (new_role.as_u64 == context->mmu_role.as_u64 &&
> +	    context->root_level == new_root_level)
>  		return;
>  
>  	context->mmu_role.as_u64 = new_role.as_u64;
> @@ -4817,8 +4823,8 @@ static void shadow_mmu_init_context(struct kvm_vcpu *vcpu, struct kvm_mmu *conte
>  		paging64_init_context(context);
>  	else
>  		paging32_init_context(context);
> -	context->root_level = role_regs_to_root_level(regs);
>  
> +	context->root_level = new_root_level;
>  	reset_guest_paging_metadata(vcpu, context);
>  	context->shadow_root_level = new_role.base.level;

-- 
Vitaly