On Mon, Dec 18, 2023 at 6:08 AM Tao Su <tao1.su@xxxxxxxxxxxxxxx> wrote: > > With 4-level EPT, bits 51:48 of the guest physical address must all > be zero; otherwise, an EPT violation always occurs, which is an unexpected > VM exit in KVM currently. > > Even though KVM advertises the max physical bits to guest, guest may > ignore MAXPHYADDR in CPUID and set a bigger physical bits to KVM. > Rejecting invalid guest physical bits on KVM side is a choice, but it will > break current KVM ABI, e.g., current QEMU ignores the physical bits > advertised by KVM and uses host physical bits as guest physical bits by > default when using '-cpu host', although we would like to send a patch to > QEMU, it will still cause backward compatibility issues. > > For GPA that can't be translated by EPT but within host.MAXPHYADDR, > emulation should be the best choice since KVM will inject #PF for the > invalid GPA in guest's perspective and try to emulate the instructions > which minimizes the impact on guests as much as possible. > > Signed-off-by: Tao Su <tao1.su@xxxxxxxxxxxxxxx> > Tested-by: Yi Lai <yi1.lai@xxxxxxxxx> > --- > arch/x86/kvm/vmx/vmx.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index be20a60047b1..a8aa2cfa2f5d 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -5774,6 +5774,13 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu) > > vcpu->arch.exit_qualification = exit_qualification; > > + /* > + * Emulate the instruction when accessing a GPA which is set any bits > + * beyond guest-physical bits that EPT can translate. > + */ > + if (unlikely(gpa & rsvd_bits(kvm_mmu_tdp_maxphyaddr(), 63))) > + return kvm_emulate_instruction(vcpu, 0); > + This doesn't really work, since the KVM instruction emulator is woefully incomplete. To make this work, first you have to teach the KVM instruction emulator how to emulate *all* memory-accessing instructions. > /* > * Check that the GPA doesn't exceed physical memory limits, as that is > * a guest page fault. We have to emulate the instruction here, because > -- > 2.34.1 > >
