On 20/02/2018 12:22, David Woodhouse wrote: >>>> However, Paolo is very insistent that taking the trap every time is >>>> actually a lot *slower* than really frobbing IBRS on certain >>>> microarchitectures, so my hand-waving "pfft, what did they expect?" is >>>> not acceptable. >>>> >>>> Which I think puts us back to the "throwing the toys out of the pram" >> There are no more toys in the pram. I threw them all out weeks ago ... > > One option is to take the patch as-is¹ with the trap on every access. Please reword the commit message at least, mentioning that the slow case is not one we don't care much about yet (no IBRS_ALL CPUs in the wild afaik) and we won't care much about in the long run either (IBRS_ALL really only used on a handful of blacklisted processors). Thanks, Paolo > As soon as Intel define that 'IBRS_ALL_AND_THE_BIT_IS_A_NOOP' bit in > MSR_IA32_ARCH_CAPABILITIES, *then* we can expose it to guests directly > again just as we do at the moment. > > That way, the slowdown that Paolo is concerned about is limited to a > small set of current CPUs on which we're mostly unlikely to care too > much about KVM anyway.
