Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest
To: Liran Alon <liran.alon@oracle.com>
Cc: jmattson@google.com, x86@kernel.org, dwmw@amazon.co.uk,
        bp@alien8.de, aliguori@amazon.com, thomas.lendacky@amd.com,
        rkrcmar@redhat.com, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
References: <222d0a6b-820f-4d7c-a616-ac89f77c3c09@default>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <834e9b46-5d59-a81e-8cda-5f576964e1cb@redhat.com>
Date: Tue, 9 Jan 2018 17:57:08 +0100
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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