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SubjectRe: [PATCH v6 1/7] KVM: x86: Deflect unknown MSR accesses to user space
> >> +The "reason" field specifies why the MSR trap occurred. User space will only
> >> +receive MSR exit traps when a particular reason was requested during through
> >> +ENABLE_CAP. Currently valid exit reasons are:
> >> +
> >> + KVM_MSR_EXIT_REASON_INVAL - access to invalid MSRs or reserved bits
> >
> >
> > Can we also have ENOENT?
> I tried to add that at first, but it gets tricky really fast. Why should
> user space have a vested interest in differentiating between "MSR is not
> implemented" and "MSR is guarded by a CPUID flag and thus not handled"
> or "MSR is guarded by a CAP"?
> The more details we reveal, the more likely we're to break ABI
> compatibility.

I don't suspect we will ever have a stable ABI here, whether we split
the two error values or not. But there could be value in it.
Consider an MSR that raises #GP if any bit in the high dword is set.
KVM version 0 knows nothing about this MSR, but KVM version 1
implements it properly. Assuming ignore_msrs=0 and error codes:
EINVAL (invalid argument, should raise a #GP) and ENOTSUP (seems like
a better name than ENOENT, Unknown MSR).

With Just EINVAL: KVM version 0 will always exit to userspace if
EINVAL is requested (it needs help). KVM version 1 will exit to
userspace only for illegal accesses if EINVAL is requested (but it
doesn't really need help).
With EINVAL and ENOTSUP: KVM version 0 will always exit to userspace
if ENOTSUP is requested (it needs help). KVM version 1 will not exit
to userspace if ENOTSUP is requested.

If you want to implement ignore_msrs in userspace it seems much easier
with the second approach, and I think all you'd have to do is return
-ENOTSUP from kvm_msr_ignored_check() instead of returning 1.

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