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SubjectRe: Avoid speculative indirect calls in kernel
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On 04/01/2018 15:51, Andrew Cooper wrote:
> Where have you got this idea from?  Using IBPB on every mode switch
> would be an insane overhead to take, and isn't necessary.

IIRC it started as a paranoia mode for AMD, but then we found out it was
actually faster than IBRS on some Intel processor where IBRS performance
was horrible. But I don't remember the details of the performance
testing, sorry.

Paolo

> Also, remember that PTI and these mitigations are for orthogonal issues.
>
> Perhaps it is easiest to refer directly to the Xen SP2 mitigations and
> my commentary of what is going on:
> http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=blob;f=xen/arch/x86/spec_ctrl.c;h=79aedf774a390293dfd564ce978500085344e305;hb=refs/heads/sp2-mitigations-v6.5#l192
>
> With the GCC -mindirect-branch=thunk-external support, and microcode,
> Xen will make a boot-time choice between using Retpoline, Lfence (which
> is the better AMD option, and more performant than retpoline), or IBRS
> on Skylake and newer processors where it is strictly necessary, as well
> as using IBPB whenever available.
>
> It also supports virtualising IBRS for guest usage when the kernel has
> chosen not to use it; a configuration I haven't seen in any of the Linux
> patch series thusfar.
>
> ~Andrew
>

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