Re: Avoid speculative indirect calls in kernel

["Woodhouse, David" <dwmw@amazon.co.uk>]

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On Thu, 2018-01-04 at 14:51 +0000, Andrew Cooper wrote:
> 
> > * never turn off indirect branch prediction, but use a branch prediction
> > barrier on every mode switch (needed for current AMD microcode)
> 
> Where have you got this idea from?  Using IBPB on every mode switch
> would be an insane overhead to take, and isn't necessary.

AMD *only* has IBPB and not IBRS, but IIRC you don't need to do it on
every context switch into the kernel; only when switching between
VMs/processes?

> 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.

I need to pull in the AMD lfence alternative for retpoline, giving us a
3-way choice of the existing retpoline thunk, "lfence; jmp *%\reg", and
a bare "jmp *%\reg".

Then the IBRS bits can be added on top.

> 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.

Adding that for KVM is in the Linux IBRS patch set that I've seen.
Didn't we already have a conversation about how the Linux patch set
does it as an atomically-switched MSR while you've done it manually in
Xen because it's faster?

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