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

From: Sean Christopherson <seanjc@google.com>
To: Wei Huang <wei.huang2@amd.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com,
	joro@8bytes.org, tglx@linutronix.de, mingo@redhat.com,
	bp@alien8.de, x86@kernel.org, hpa@zytor.com
Subject: Re: [PATCH v3 0/3] SVM 5-level page table support
Date: Mon, 23 Aug 2021 16:10:40 +0000	[thread overview]
Message-ID: <YSPIgBNiMZkwAOSG@google.com> (raw)
In-Reply-To: <20210823151549.rkkrktvtpu6yapmd@weiserver.amd.com>

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?

        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;
> >  
> >         /*
> > 
> > 
> > 
> > Best regards,
> > 	Maxim Levitsky
> >
