linux-kernel - Re: Revert "x86/speculation: Simplify indirect_branch_prediction

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Date:   Thu, 15 Feb 2018 23:28:30 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Richard Weinberger <richard@...ma-star.at>
Cc:     mingo@...nel.org, luto@...nel.org, arjan@...ux.intel.com,
        bp@...en8.de, dan.j.williams@...el.com,
        dave.hansen@...ux.intel.com, gregkh@...uxfoundation.org,
        jpoimboe@...hat.com, torvalds@...ux-foundation.org,
        peterz@...radead.org, tglx@...utronix.de,
        arjan.van.de.ven@...el.com, dave.hansen@...el.com,
        jmattson@...gle.com, karahmed@...zon.de, kvm@...r.kernel.org,
        pbonzini@...hat.com, rkrcmar@...hat.com, sironi@...zon.de,
        linux-kernel@...r.kernel.org
Subject: Re: Revert "x86/speculation: Simplify
 indirect_branch_prediction_barrier()" crashes KVM guest

On Fri, 2018-02-16 at 00:12 +0100, Richard Weinberger wrote:
> 
> [    2.791518] Code: 8b 45 00 49 8b 7d 08 49 83 c5 18 31 d2 31 f6 ff
> d0 49 8b 45 00 48 85 c0 75 e9 eb b1 b9 49 00 00 00 b8 01 00 00 00 ba
> 00 00 00 00 <0f> 30 e9 68 fd ff ff 9c 58 0f 1f 44 00 00 48 89 c5 fa
> 66 0f 1f

  23:   b9 49 00 00 00          mov    $0x49,%ecx
  28:   b8 01 00 00 00          mov    $0x1,%eax
  2d:   ba 00 00 00 00          mov    $0x0,%edx
  32:   0f 30                   wrmsr  

The faulting instruction is the wrmsr itself. That shouldn't happen; if
the CPUID bit indicates that the MSR exists, then it should exist.

The reverted patch did use the C __wrmsr() macro which contained a
fixup for this GP# but it was just a side-effect of the "cleanup" — it
wasn't intentional because that really shouldn't happen. That looks
like a qemu bug as first glance.
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