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From: Sean Christopherson <seanjc@google.com>
To: Ben Gardon <bgardon@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots
Date: Wed, 3 Mar 2021 08:46:59 -0800	[thread overview]
Message-ID: <YD+9gwhfFCS7Xghe@google.com> (raw)
In-Reply-To: <CANgfPd95G-01ObzeKeMTUu0yXBJ=0+ZGQwr_5WCNH-NmR03f9w@mail.gmail.com>

On Tue, Mar 02, 2021, Ben Gardon wrote:
> > @@ -3241,16 +3237,10 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
> >
> >         if (is_tdp_mmu_enabled(vcpu->kvm)) {
> >                 root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
> > -
> > -               if (!VALID_PAGE(root))
> > -                       return -ENOSPC;
> >                 vcpu->arch.mmu->root_hpa = root;
> >         } else if (shadow_root_level >= PT64_ROOT_4LEVEL) {
> >                 root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level,
> >                                       true);
> > -
> > -               if (!VALID_PAGE(root))
> > -                       return -ENOSPC;
> 
> There's so much going on in mmu_alloc_root that removing this check
> makes me nervous, but I think it should be safe.

Just think of it as a variant of kvm_mmu_get_page(), then all your fears will
melt away. ;-)

> I checked though the function because I was worried it might yield
> somewhere in there, which could result in the page cache being emptied
> and the allocation failing, but I don't think mmu_alloc_root this
> function will yield.

Ugh, mmu_alloc_root() won't yield, but get_zeroed_page() used to allocate pae_root
and lm_root on-demand in mmu_alloc_shadow_roots() will.  The two options are
(a) allocate the fake roots before taking the lock and (b) allocate them from
vcpu->arch.mmu_shadow_page_cache.  I probably prefer (a).  (b) will slide
directly into the existing code, but would require bumping the min number of
objects for mmu_shadow_page_cache in mmu_topup_memory_caches().  I'd prefer not
to have yet more code that has to deal with this insanity.

  reply	other threads:[~2021-03-03 19:28 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-02 18:45 [PATCH 00/15] KVM: x86/mmu: Lots of bug fixes Sean Christopherson
2021-03-02 18:45 ` [PATCH 01/15] KVM: nSVM: Set the shadow root level to the TDP level for nested NPT Sean Christopherson
2021-03-02 18:45 ` [PATCH 02/15] KVM: x86/mmu: Alloc page for PDPTEs when shadowing 32-bit NPT with 64-bit Sean Christopherson
2021-03-03 17:28   ` Ben Gardon
2021-03-02 18:45 ` [PATCH 03/15] KVM: x86/mmu: Ensure MMU pages are available when allocating roots Sean Christopherson
2021-03-03  0:21   ` Ben Gardon
2021-03-03 16:46     ` Sean Christopherson [this message]
2021-03-02 18:45 ` [PATCH 04/15] KVM: x86/mmu: Allocate the lm_root before allocating PAE roots Sean Christopherson
2021-03-02 18:45 ` [PATCH 05/15] KVM: x86/mmu: Check PDPTRs " Sean Christopherson
2021-03-02 18:45 ` [PATCH 06/15] KVM: x86/mmu: Fix and unconditionally enable WARNs to detect PAE leaks Sean Christopherson
2021-03-02 18:45 ` [PATCH 07/15] KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE root Sean Christopherson
2021-03-02 18:45 ` [PATCH 08/15] KVM: x86/mmu: Set the C-bit in the PDPTRs and LM pseudo-PDPTRs Sean Christopherson
2021-03-02 18:45 ` [PATCH 09/15] KVM: x86/mmu: Mark the PAE roots as decrypted for shadow paging Sean Christopherson
2021-03-02 18:45 ` [PATCH 10/15] KVM: SVM: Don't strip the C-bit from CR2 on #PF interception Sean Christopherson
2021-03-02 18:45 ` [PATCH 11/15] KVM: nVMX: Defer the MMU reload to the normal path on an EPTP switch Sean Christopherson
2021-03-02 18:45 ` [PATCH 12/15] KVM: x86: Defer the MMU unload to the normal path on an global INVPCID Sean Christopherson
2021-03-02 18:45 ` [PATCH 13/15] KVM: x86/mmu: Unexport MMU load/unload functions Sean Christopherson
2021-03-02 18:45 ` [PATCH 14/15] KVM: x86/mmu: Sync roots after MMU load iff load as successful Sean Christopherson
2021-03-02 18:45 ` [PATCH 15/15] KVM: x86/mmu: WARN on NULL pae_root and bad shadow root level Sean Christopherson

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