Re: [PATCH 05/54] Revert "KVM: x86/mmu: Drop kvm_mmu_extended_role.cr4_la57 hack"

[Yu Zhang]

On Tue, Jun 22, 2021 at 10:56:50AM -0700, Sean Christopherson wrote:
> Restore CR4.LA57 to the mmu_role to fix an amusing edge case with nested
> virtualization.  When KVM (L0) is using TDP, CR4.LA57 is not reflected in
> mmu_role.base.level because that tracks the shadow root level, i.e. TDP
> level.  Normally, this is not an issue because LA57 can't be toggled
> while long mode is active, i.e. the guest has to first disable paging,
> then toggle LA57, then re-enable paging, thus ensuring an MMU
> reinitialization.
> 
> But if L1 is crafty, it can load a new CR4 on VM-Exit and toggle LA57
> without having to bounce through an unpaged section.  L1 can also load a

May I ask how this is done by the guest? Thanks!

> new CR3 on exit, i.e. it doesn't even need to play crazy paging games, a
> single entry PML5 is sufficient.  Such shenanigans are only problematic
> if L0 and L1 use TDP, otherwise L1 and L2 share an MMU that gets
> reinitialized on nested VM-Enter/VM-Exit due to mmu_role.base.guest_mode.
> 
> Note, in the L2 case with nested TDP, even though L1 can switch between
> L2s with different LA57 settings, thus bypassing the paging requirement,
> in that case KVM's nested_mmu will track LA57 in base.level.
> 
> This reverts commit 8053f924cad30bf9f9a24e02b6c8ddfabf5202ea.
> 
> Fixes: 8053f924cad3 ("KVM: x86/mmu: Drop kvm_mmu_extended_role.cr4_la57 hack")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 1 +
>  arch/x86/kvm/mmu/mmu.c          | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index e11d64aa0bcd..916e0f89fdfc 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -320,6 +320,7 @@ union kvm_mmu_extended_role {
>  		unsigned int cr4_pke:1;
>  		unsigned int cr4_smap:1;
>  		unsigned int cr4_smep:1;
> +		unsigned int cr4_la57:1;
>  		unsigned int maxphyaddr:6;
>  	};
>  };
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0db12f461c9d..5024318dec45 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4537,6 +4537,7 @@ static union kvm_mmu_extended_role kvm_calc_mmu_role_ext(struct kvm_vcpu *vcpu)
>  	ext.cr4_smap = !!kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
>  	ext.cr4_pse = !!is_pse(vcpu);
>  	ext.cr4_pke = !!kvm_read_cr4_bits(vcpu, X86_CR4_PKE);
> +	ext.cr4_la57 = !!kvm_read_cr4_bits(vcpu, X86_CR4_LA57);
>  	ext.maxphyaddr = cpuid_maxphyaddr(vcpu);
>  
>  	ext.valid = 1;
> -- 
> 2.32.0.288.g62a8d224e6-goog
> 

B.R.
Yu