linux-kernel - Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Tue, 9 Jan 2018 17:57:08 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     jmattson@...gle.com, x86@...nel.org, dwmw@...zon.co.uk,
        bp@...en8.de, aliguori@...zon.com, thomas.lendacky@....com,
        rkrcmar@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On 09/01/2018 17:48, Liran Alon wrote:
>>>  
>>> +	if (have_spec_ctrl) {
>>> +		rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>>> +		if (vmx->spec_ctrl != 0)
>>> +			wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>
> As I said also on the AMD patch, I think this is a bug.
> Intel specify that we should set IBRS bit even if it was already set on every #VMExit.

That's correct (though I'd like to understand _why_---I'm not inclined
to blindly trust a spec), but for now it's saving a wrmsr of 0.  That is
quite obviously okay, and will be also okay after the bare-metal IBRS
patches.

Of course the code will become something like

	if (using_ibrs || vmx->spec_ctrl != 0)
		wrmsrl(MSR_IA32_SPEC_CTRL, host_ibrs);

optimizing the case where the host is using retpolines.

Paolo