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Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL
From:   David Woodhouse <dwmw2@infradead.org>
To:     KarimAllah Ahmed <karahmed@amazon.com>,
        Liran Alon <liran.alon@oracle.com>
Cc:     konrad.wilk@oracle.com, luto@kernel.org, tglx@linutronix.de,
        torvalds@linux-foundation.org, gregkh@linuxfoundation.org,
        asit.k.mallick@intel.com, dave.hansen@intel.com,
        karahmed@amazon.de, jun.nakajima@intel.com,
        dan.j.williams@intel.com, ashok.raj@intel.com,
        daniel.kiper@oracle.com, arjan.van.de.ven@intel.com,
        tim.c.chen@linux.intel.com, pbonzini@redhat.com,
        linux-kernel@vger.kernel.org, ak@linux.intel.com,
        kvm@vger.kernel.org, aarcange@redhat.com
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Date:   Mon, 29 Jan 2018 10:37:44 +0000
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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*.
>=20
> 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

=C2=A0/* If this vCPU has touched SPEC_CTRL then restore its value if neede=
d */
=C2=A0if (vmx->spec_ctrl_live && vmx->spec_ctrl)
=C2=A0 =C2=A0 =C2=A0wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
=C2=A0/* 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 ha=
ve 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?=C2=A0
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