Messages in this thread | | | Date | Fri, 29 Apr 2022 21:32:33 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v3] x86/speculation, KVM: only IBPB for switch_mm_always_ibpb on vCPU load |
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On Fri, Apr 29, 2022 at 05:31:16PM +0000, Jon Kohler wrote: > Selftests IIUC, but there may be other VMMs that do funny stuff. Said > another way, I don’t think we actively restrict user space from doing > this as far as I know.
"selftests", "there may be"?!
This doesn't sound like a real-life use case to me and we don't do changes just because. Sorry.
> The paranoid aspect here is KVM is issuing an *additional* IBPB on > top of what already happens in switch_mm().
Yeah, I know how that works.
> IMHO KVM side IBPB for most use cases isn’t really necessarily but > the general concept is that you want to protect vCPU from guest A > from guest B, so you issue a prediction barrier on vCPU switch. > > *however* that protection already happens in switch_mm(), because > guest A and B are likely to use different mm_struct, so the only point > of having this support in KVM seems to be to “kill it with fire” for > paranoid users who might be doing some tomfoolery that would > somehow bypass switch_mm() protection (such as somehow > sharing a struct).
Yeah, no, this all sounds like something highly hypothetical or there's a use case of which you don't want to talk about publicly.
Either way, from what I'm reading I'm not in the least convinced that this is needed.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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