linux-kernel - Re: [PATCH v3 0/3] SVM 5-level page table support

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Date:   Mon, 23 Aug 2021 21:10:46 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Wei Huang <wei.huang2@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com
Subject: Re: [PATCH v3 0/3] SVM 5-level page table support

On Mon, 2021-08-23 at 16:10 +0000, Sean Christopherson wrote:
> On Mon, Aug 23, 2021, Wei Huang wrote:
> > On 08/23 12:20, Maxim Levitsky wrote:
> > > This hack makes it work again for me (I don't yet use TDP mmu).
> > > 
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index caa3f9aee7d1..c25e0d40a620 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -3562,7 +3562,7 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
> > >             mmu->shadow_root_level < PT64_ROOT_4LEVEL)
> > >                 return 0;
> > >  
> > > -       if (mmu->pae_root && mmu->pml4_root && mmu->pml5_root)
> 
> Maxim, I assume you hit this WARN and bail?
Yep.
> 
>         if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root ||
>                          mmu->pml5_root))
> 		return -EIO;
> 
> Because as the comment states, KVM expects all the special roots to be allocated
> together.  The 5-level paging supported breaks that assumption because pml5_root
> will be allocated iff the host is using 5-level paging.
> 
>         if (mmu->shadow_root_level > PT64_ROOT_4LEVEL) {
>                 pml5_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
>                 if (!pml5_root)
>                         goto err_pml5;
>         }
> 
> I think this is the least awful fix, I'll test and send a proper patch later today.
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4853c033e6ce..93b2ed422b48 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3548,6 +3548,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
>  {
>         struct kvm_mmu *mmu = vcpu->arch.mmu;
> +       bool need_pml5 = mmu->shadow_root_level > PT64_ROOT_4LEVEL;
>         u64 *pml5_root = NULL;
>         u64 *pml4_root = NULL;
>         u64 *pae_root;
> @@ -3562,7 +3563,14 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
>             mmu->shadow_root_level < PT64_ROOT_4LEVEL)
>                 return 0;
> 
> -       if (mmu->pae_root && mmu->pml4_root && mmu->pml5_root)
> +       /*
> +        * NPT, the only paging mode that uses this horror, uses a fixed number
> +        * of levels for the shadow page tables, e.g. all MMUs are 4-level or
> +        * all MMus are 5-level.  Thus, this can safely require that pml5_root
> +        * is allocated if the other roots are valid and pml5 is needed, as any
> +        * prior MMU would also have required pml5.
> +        */
> +       if (mmu->pae_root && mmu->pml4_root && (!need_pml5 || mmu->pml5_root))
>                 return 0;
> 
>         /*
> @@ -3570,7 +3578,7 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
>          * bail if KVM ends up in a state where only one of the roots is valid.
>          */
>         if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root ||
> -                        mmu->pml5_root))
> +                        (need_pml5 && mmu->pml5_root)))
>                 return -EIO;
> 
>         /*
> 
> > > +       if (mmu->pae_root && mmu->pml4_root)
> > >                 return 0;
> > >  
> > >         /*

Makes sense, works, and without digging too much into this
I expected this to be fixed by something like that, so:

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Thanks,
Best regards,
	Maxim Levitsky



> > > 
> > > 
> > > 
> > > Best regards,
> > > 	Maxim Levitsky
> > >