Re: [PATCH 17/15] KVM: X86: Ensure pae_root to be reconstructed for shadow paging if the guest PDPTEs is changed

Lai Jiangshan
2021-12-08T04:00:45Z

On 2021/12/8 08:15, Sean Christopherson wrote:
>>
>> The commit 21823fbda552("KVM: x86: Invalidate all PGDs for the current
>> PCID on MOV CR3 w/ flush") skips kvm_mmu_free_roots() after
>> load_pdptrs() when rewriting the CR3 with the same value.
>
> This isn't accurate, prior to that commit KVM wasn't guaranteed to do
> kvm_mmu_free_roots() if it got a hit on the current CR3 or if a previous CR3 in
> the cache matched the new CR3 (the "cache" has done some odd things in the past).
>
> So I think this particular flavor would be:
>
> Fixes: 7c390d350f8b ("kvm: x86: Add fast CR3 switch code path")

If guest is 32bit, fast_cr3_switch() always return false, and
kvm_mmu_free_roots() is always called, and no cr3 goes in prev_root.

And from 21823fbda552, fast_cr3_switch() and kvm_mmu_free_roots() are
both skipped when cr3 is unchanged.

>
>> The commit a91a7c709600("KVM: X86: Don't reset mmu context when
>> toggling X86_CR4_PGE") skips kvm_mmu_reset_context() after
>> load_pdptrs() when changing CR4.PGE.
>>
>> Normally, the guest doesn't change the PDPTEs before doing only the
>> above operation without touching other bits that can force pae_root to
>> be reconstructed. Guests like linux would keep the PDPTEs unchaged
>> for every instance of pagetable.
>>
>> Fixes: d81135a57aa6("KVM: x86: do not reset mmu if CR0.CD and CR0.NW are changed")
>> Fixes: 21823fbda552("KVM: x86: Invalidate all PGDs for the current PCID on MOV CR3 w/ flush")
>> Fixes: a91a7c709600("KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE")
>> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
>> ---
>> arch/x86/kvm/x86.c | 10 ++++++++--
>> 1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 0176eaa86a35..cfba337e46ab 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -832,8 +832,14 @@ int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3)
>> if (memcmp(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs))) {
>> memcpy(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs));
>> kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
>> - /* Ensure the dirty PDPTEs to be loaded. */
>> - kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
>> + /*
>> + * Ensure the dirty PDPTEs to be loaded for VMX with EPT
>> + * enabled or pae_root to be reconstructed for shadow paging.
>> + */
>> + if (tdp_enabled)
>> + kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
>> + else
>> + kvm_mmu_free_roots(vcpu, vcpu->arch.mmu, KVM_MMU_ROOT_CURRENT);
>
> Shouldn't matter since it's legacy shadow paging, but @mmu should be used instead
> of vcpu->arch.mmuvcpu->arch.mmu.

@mmu is the "guest mmu" (vcpu->arch.walk_mmu), which is used to walk
including loading pdptr.

vcpu->arch.mmu is for host constructing mmu for shadowed or tdp mmu
which is used in host side management including kvm_mmu_free_roots().

Even they are the same pointer now for !tdp, the meaning is different. I prefer
to distinguish them even before kvm_mmu is split different for guest mmu
(vcpu->arch.walk_mmu) and host constructing mmu (vcpu->arch.mmu).

(I once searched all the usage of undistinguished usage of kvm_mmu *mmu, and
found a bug, see "Use vcpu->arch.walk_mmu for kvm_mmu_invlpg()")

I think Paolo is doing the splitting, unless I would take the job because
I have some patches pending depended them.

>
> To avoid a dependency on the previous patch, I think it makes sense to have this be:
>
> if (!tdp_enabled && memcmp(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs)))
> kvm_mmu_free_roots(vcpu, mmu, KVM_MMU_ROOT_CURRENT);
>

Yes, it is a good idea to add this first.

Thanks for review and suggestion.
Lai