Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs

[Paolo Bonzini <pbonzini@redhat.com>]

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.