Re: [PATCH v2 01/18] KVM: x86: host-initiated EFER.LME write affects the MMU

Sean Christopherson
2022-02-18T17:08:17Z

The shortlog doesn't come remotely close to saying what this patch does, it's
simply a statement.

KVM: x86: Reset the MMU context if host userspace toggles EFER.LME

On Thu, Feb 17, 2022, Paolo Bonzini wrote:
> While the guest runs, EFER.LME cannot change unless CR0.PG is clear, and therefore
> EFER.NX is the only bit that can affect the MMU role. However, set_efer accepts
> a host-initiated change to EFER.LME even with CR0.PG=1. In that case, the
> MMU has to be reset.

Wrap at ~75 please.

> Fixes: 11988499e62b ("KVM: x86: Skip EFER vs. guest CPUID checks for host-initiated writes")
> Cc: stable@vger.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

With nits addressed,

Reviewed-by: Sean Christopherson <seanjc@google.com>

> arch/x86/kvm/mmu.h | 1 +
> arch/x86/kvm/x86.c | 2 +-
> 2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 51faa2c76ca5..a5a50cfeffff 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -48,6 +48,7 @@
> X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE)
>
> #define KVM_MMU_CR0_ROLE_BITS (X86_CR0_PG | X86_CR0_WP)
> +#define KVM_MMU_EFER_ROLE_BITS (EFER_LME | EFER_NX)
>
> static __always_inline u64 rsvd_bits(int s, int e)
> {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d3da64106685..99a58c25f5c2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1647,7 +1647,7 @@ static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> }
>
> /* Update reserved bits */

This comment needs to be dropped, toggling EFER.LME affects more than just reserved
bits.

> - if ((efer ^ old_efer) & EFER_NX)
> + if ((efer ^ old_efer) & KVM_MMU_EFER_ROLE_BITS)
> kvm_mmu_reset_context(vcpu);
>
> return 0;
> --
> 2.31.1
>
>