linux-kernel - Re: [PATCH v3 4/4] KVM: VMX: Allow direct access to MSR_IA32_SPEC

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Message-ID: <CALMp9eQNTA3p7BF_r3BECrtSgz49q7J0-caW_PFjRAou-7AcKQ@mail.gmail.com>
Date:   Tue, 30 Jan 2018 16:16:14 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     KarimAllah Ahmed <karahmed@...zon.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.de>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v3 4/4] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

On Tue, Jan 30, 2018 at 3:50 PM, KarimAllah Ahmed <karahmed@...zon.com> wrote:
> Okay, I took a second look at the code (specially
> nested_vmx_merge_msr_bitmap).
>
> This means that I simply should not touch the MSR bitmap in set_msr in
> case of nested, I just need to properly update the l02 msr_bitmap in
> nested_vmx_merge_msr_bitmap. As in here:
>
> http://git.infradead.org/linux-retpoline.git/commitdiff/d90eedebdd16bb00741a2c93bc13c5e444c99c2b
>
> or am I still missing something? (sorry, did not actually look at the
> nested code before!)

+               if (cpu_has_vmx_msr_bitmap() && data &&
+                   !vmx->save_spec_ctrl_on_exit) {
+                       vmx->save_spec_ctrl_on_exit = true;
+
+                       if (is_guest_mode(vcpu))
+                               break;

As Paolo suggested, the test for !is_guest_mode (vcpu) should just be
folded into the condition above. If you aren't clearing a 'W' bit in
the MSR permission bitmap, there's no need to set
vmx->save_spec_ctrl_on_exit.

+
+                       vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap,
+                                                     MSR_IA32_SPEC_CTRL,
+                                                     MSR_TYPE_RW);
+               }
+               break;

 ...

+       if (guest_cpuid_has(vcpu, X86_FEATURE_IBRS)) {
+               nested_vmx_disable_intercept_for_msr(
+                               msr_bitmap_l1, msr_bitmap_l0,
+                               MSR_IA32_SPEC_CTRL,
+                               MSR_TYPE_R | MSR_TYPE_W);
+       }
+

However, here, you should set vmx->save_spec_ctrl_on_exit if
nested_vmx_disable_intercept_for_msr clears the 'W' bit for
MSR_IA32_SPEC_CTRL in msr_bitmap_l0. Perhaps this would be easier if
nested_vmx_disable_intercept_for_msr returned something indicative of
which bits it cleared (if any).