linux-kernel - Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC

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Date:   Mon, 29 Jan 2018 12:27:41 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     David Woodhouse <dwmw2@...radead.org>, daniel.kiper@...cle.com,
        Mihai Carabas <mihai.carabas@...cle.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.com>,
        Liran Alon <liran.alon@...cle.com>, luto@...nel.org,
        tglx@...utronix.de, torvalds@...ux-foundation.org,
        gregkh@...uxfoundation.org, asit.k.mallick@...el.com,
        dave.hansen@...el.com, karahmed@...zon.de, jun.nakajima@...el.com,
        dan.j.williams@...el.com, ashok.raj@...el.com,
        daniel.kiper@...cle.com, arjan.van.de.ven@...el.com,
        tim.c.chen@...ux.intel.com, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kvm@...r.kernel.org, aarcange@...hat.com
Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

On Mon, Jan 29, 2018 at 10:37:44AM +0000, David Woodhouse wrote:
> 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
> 
>  /* If this vCPU has touched SPEC_CTRL then restore its value if needed */
>  if (vmx->spec_ctrl_live && vmx->spec_ctrl)
>      wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>  /* 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? 

Or the cpuid_flag as that would determine whether the MSR bitmap intercept
is set or not.