Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

[David Woodhouse]

On Mon, 2018-01-29 at 10:43 +0100, KarimAllah Ahmed wrote:
> On 01/29/2018 09:46 AM, David Woodhouse wrote:
> > Reading the code and comparing with the SDM, I can't see where we're
> > ever setting VM_EXIT_MSR_STORE_{ADDR,COUNT} except in the nested
> > case...
> Hmmm ... you are probably right! I think all users of this interface
> always trap + update save area and never passthrough the MSR. That is
> why only LOAD is needed *so far*.
> 
> Okay, let me sort this out in v3 then.

I'm starting to think a variant of Ashok's patch might actually be the
simpler approach, and not "premature optimisation". Especially if we
need to support the !cpu_has_vmx_msr_bitmaps() case?

Start with vmx->spec_ctrl set to zero. When first touched, make it
passthrough (but not atomically switched) and set a flag (e.g.
"spec_ctrl_live") which triggers the 'restore_branch_speculation' and
'save_and_restrict_branch_speculation' behaviours. Except don't use
those macros. Those can look something like

Â/* If this vCPU has touched SPEC_CTRL then restore its value if needed */
Âif (vmx->spec_ctrl_live && vmx->spec_ctrl)
  Âwrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
Â/* vmentry is serialising on affected CPUs, so the conditional branch is safe */


... and, respectively, ...

 /* If this vCPU has touched SPEC_CTRL then save its value and ensure we have zero */
 if (vmx->spec_ctrl_live) {
     rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
     if (vmx->spec_ctrl)
         wrmsrl(MSR_IA32_SPEC_CTRL, 0);
 }


Perhaps we can ditch the separate 'spec_ctrl_live' flag and check the
pass-through MSR bitmap directly, in the case that it exists?Â

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