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 21:03:27 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Liran Alon <liran.alon@...cle.com>, 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

Paolo Bonzini <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.

Excuse my ignorance: Can you point me to the specifications that mention “we
should set IBRS bit even if it was already set on every #VMExit” ?

Thanks,
Nadav