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Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support
To: Jim Mattson <jmattson@google.com>, KarimAllah Ahmed <karahmed@amazon.de>
Cc: the arch/x86 maintainers <x86@kernel.org>,
 LKML <linux-kernel@vger.kernel.org>, kvm list <kvm@vger.kernel.org>,
 Ashok Raj <ashok.raj@intel.com>, Asit Mallick <asit.k.mallick@intel.com>,
 Dave Hansen <dave.hansen@intel.com>,
 Arjan Van De Ven <arjan.van.de.ven@intel.com>,
 Tim Chen <tim.c.chen@linux.intel.com>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Andrea Arcangeli <aarcange@redhat.com>, Andi Kleen <ak@linux.intel.com>,
 Thomas Gleixner <tglx@linutronix.de>, Dan Williams
 <dan.j.williams@intel.com>, Jun Nakajima <jun.nakajima@intel.com>,
 Andy Lutomirski <luto@kernel.org>, Greg KH <gregkh@linuxfoundation.org>,
 Peter Zijlstra <peterz@infradead.org>, David Woodhouse <dwmw@amazon.co.uk>
References: <1517404231-22406-1-git-send-email-karahmed@amazon.de>
 <1517404231-22406-3-git-send-email-karahmed@amazon.de>
 <CALMp9eRKz+sKeah0V0bpiYJJUgTM5oisbtahkNGtW=seW2W5JQ@mail.gmail.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <7dbf6cad-f03d-640c-bc95-d82deaa30cb7@redhat.com>
Date: Wed, 31 Jan 2018 11:55:52 -0500
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On 31/01/2018 11:50, Jim Mattson wrote:
>> +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
>> +               nested_vmx_disable_intercept_for_msr(
>> +                               msr_bitmap_l1, msr_bitmap_l0,
>> +                               MSR_IA32_PRED_CMD,
>> +                               MSR_TYPE_R);
>> +       }
> I don't think this should be predicated on
> "to_vmx(vcpu)->save_spec_ctrl_on_exit." Why not just
> "guest_cpuid_has(vcpu, X86_FEATURE_IBPB)"? Also, the final argument to
> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
> MSR_TYPE_R.

In fact this MSR can even be passed down unconditionally, since it needs
no save/restore and has no ill performance effect on the sibling
hyperthread.

Only MSR_IA32_SPEC_CTRL needs to be conditional on
"to_vmx(vcpu)->save_spec_ctrl_on_exit".

Paolo