From: Paolo Bonzini <pbonzini@redhat.com>
To: KarimAllah Ahmed <karahmed@amazon.com>,
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>
Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support
Date: Wed, 31 Jan 2018 12:15:35 -0500 [thread overview]
Message-ID: <200c5e32-e238-ee87-45ed-4701285ff489@redhat.com> (raw)
In-Reply-To: <5f37b853-e7bb-0259-653c-6f526b106594@amazon.com>
On 31/01/2018 12:11, KarimAllah Ahmed wrote:
> On 01/31/2018 05:50 PM, Jim Mattson wrote:
>> On Wed, Jan 31, 2018 at 5:10 AM, KarimAllah Ahmed <karahmed@amazon.de>
>> wrote:
>>
>>> + vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap,
>>> MSR_IA32_PRED_CMD,
>>> + MSR_TYPE_W);
>>
>> Why not disable this intercept eagerly, rather than lazily? Unlike
>> MSR_IA32_SPEC_CTRL, there is no guest value to save/restore, so there
>> is no cost to disabling the intercept if the guest cpuid info declares
>> support for it.
>>
>>
>>> + 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)"?
>
> Paolo suggested this on the previous revision because guest_cpuid_has()
> would be slow.
Sorry, that was for spec_ctrl. Here there's no need to do any kind of
conditional check.
Paolo
>> Also, the final argument to
>> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
>> MSR_TYPE_R.
>>
> Oops! will fix!
> Amazon Development Center Germany GmbH
> Berlin - Dresden - Aachen
> main office: Krausenstr. 38, 10117 Berlin
> Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
> Ust-ID: DE289237879
> Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
next prev parent reply other threads:[~2018-01-31 17:15 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-31 13:10 [PATCH v4 0/5] KVM: Expose speculation control feature to guests KarimAllah Ahmed
2018-01-31 13:10 ` [PATCH v4 1/5] KVM: x86: Update the reverse_cpuid list to include CPUID_7_EDX KarimAllah Ahmed
2018-01-31 13:10 ` [PATCH v4 2/5] KVM: x86: Add IBPB support KarimAllah Ahmed
2018-01-31 16:50 ` Jim Mattson
2018-01-31 16:55 ` Paolo Bonzini
2018-01-31 17:17 ` KarimAllah Ahmed
2018-01-31 17:39 ` Jim Mattson
2018-01-31 18:04 ` Van De Ven, Arjan
2018-01-31 18:53 ` Paolo Bonzini
2018-01-31 16:57 ` Woodhouse, David
2018-01-31 17:11 ` KarimAllah Ahmed
2018-01-31 17:15 ` Paolo Bonzini [this message]
2018-01-31 17:32 ` Paolo Bonzini
2018-01-31 13:10 ` [PATCH v4 3/5] KVM: VMX: Emulate MSR_IA32_ARCH_CAPABILITIES KarimAllah Ahmed
2018-01-31 13:10 ` [PATCH v4 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL KarimAllah Ahmed
2018-01-31 13:10 ` [PATCH v4 5/5] KVM: SVM: " KarimAllah Ahmed
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