On 19/02/2018 14:35, David Woodhouse wrote: > On Mon, 2018-02-19 at 14:10 +0100, Paolo Bonzini wrote: >>> Hardware seems like a reasonable place to get the default value (cf. >>> the VMX capability MSRs). >> >> There are some differences: >> >> - a zero value for ARCH_CAPABILITIES should be safe, while a zero value >> for VMX capabilities doesn't really make sense. On the contrary, a >> nonzero value for ARCH_CAPABILITIES is not safe across live migration. > > Any VMM which is going to support live migration surely needs to pay at > least a small amount of attention to the features it exposes? Exposing > the ARCH_CAPABILITIES CPUID bit without actually looking at the > contents of the associated MSR which that bit advertises would be... a > little strange, would it not? I think what we should do is simply backport Tom Lendacky's series to 4.14 and 4.9 ASAP, and add ARCH_CAPABILITIES support there. Then the question of the default becomes moot, more or less. Paolo > I don't see why we care so much about the *default* value, in that > context.