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 16:33:47 -0800 (PST)
From:   Liran Alon <liran.alon@...cle.com>
To:     <pbonzini@...hat.com>
Cc:     <jmattson@...gle.com>, <x86@...nel.org>, <dwmw@...zon.co.uk>,
        <bp@...en8.de>, <thomas.lendacky@....com>, <aliguori@...zon.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


----- pbonzini@...hat.com wrote:

> 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

I agree with all the above.

-Liran