On Wed, Jul 06, 2022, Maxim Levitsky wrote: > On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote: > > Deliberately truncate the exception error code when shoving it into the > > VMCS (VM-Entry field for vmcs01 and vmcs02, VM-Exit field for vmcs12). > > Intel CPUs are incapable of handling 32-bit error codes and will never > > generate an error code with bits 31:16, but userspace can provide an > > arbitrary error code via KVM_SET_VCPU_EVENTS. Failure to drop the bits > > on exception injection results in failed VM-Entry, as VMX disallows > > setting bits 31:16. Setting the bits on VM-Exit would at best confuse > > L1, and at worse induce a nested VM-Entry failure, e.g. if L1 decided to > > reinject the exception back into L2. > > Wouldn't it be better to fail KVM_SET_VCPU_EVENTS instead if it tries > to set error code with uppper 16 bits set? No, because AMD CPUs generate error codes with bits 31:16 set. KVM "supports" cross-vendor live migration, so outright rejecting is not an option. > Or if that is considered ABI breakage, then KVM_SET_VCPU_EVENTS code > can truncate the user given value to 16 bit. Again, AMD, and more specifically SVM, allows bits 31:16 to be non-zero, so truncation is only correct for VMX. I say "VMX" instead of "Intel" because architecturally the Intel CPUs do have 32-bit error codes, it's just the VMX architecture that doesn't allow injection of 32-bit values.
