Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 31 Jan 2018 14:21:02 -0800 | Subject | Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL |
| |
On Wed, Jan 31, 2018 at 2:10 PM, David Woodhouse <dwmw2@infradead.org> wrote: > > Given that IBRS_ALL is supposed to be a sanely-performing option, I'd > rather convince Intel to just make it unconditional. If they've added > the appropriate tagging to the BTB, why even *have* this deliberately > insecure mode when IBRS==0? > > I understand that until/unless they get a *proper* fix, software is > still going to have to use IBPB as appropriate. But there's no need for > the IBRS bit to do *anything*.
Amen, brother!
Please please please can Amazon and friends push this? The current situation with IBRS_ALL is complete nasty horrible garbage. It's pointless on current CPU's, and it's not well-defined enough on future CPU's.
The whole "you can enable this, but performance may or may not be acceptable, and we won't tell you" thing is some bad mumbo-jumbo.
Before IBRS is good, we'll do retpoline and BTB stuffing and have those (hopefully very rare) IBPB's. So the whole "badly performing IBRS_ALL" is completely pointless, and actively wrong.
Linus
| |