Re: [PATCH v2 02/21] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

[Sean Christopherson]

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.