KVM: x86: Exit to userspace if emulation prepared a completion callback

em_rdmsr() and em_wrmsr() return X86EMUL_IO_NEEDED if MSR accesses
required an exit to userspace. However, x86_emulate_insn() doesn't return
X86EMUL_*, so x86_emulate_instruction() doesn't directly act on
X86EMUL_IO_NEEDED; instead, it looks for other signals to differentiate
between PIO, MMIO, etc. causing RDMSR/WRMSR emulation not to
exit to userspace now.

Nevertheless, if the userspace_msr_exit_test testcase in selftests
is changed to test RDMSR/WRMSR with a forced emulation prefix,
the test passes.  What happens is that first userspace exit
information is filled but the userspace exit does not happen.
Because x86_emulate_instruction() returns 1, the guest retries
the instruction---but this time RIP has already been adjusted
past the forced emulation prefix, so the guest executes RDMSR/WRMSR
and the userspace exit finally happens.

Since the X86EMUL_IO_NEEDED path has provided a complete_userspace_io
callback, x86_emulate_instruction() can just return 0 if the
callback is not NULL. Then RDMSR/WRMSR instruction emulation will
exit to userspace directly, without the RDMSR/WRMSR vmexit.

Fixes: 1ae09954 ("KVM: x86: Allow deflecting unknown MSR accesses to user space")
Signed-off-by: Hou Wenlong <houwenlong93@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <56f9df2ee5c05a81155e2be366c9dc1f7adc8817.1635842679.git.houwenlong93@linux.alibaba.com>