linux-kernel - Re: [PATCH] kvm: x86: rewrite kvm_spec_ctrl_valid

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Date:   Tue, 7 Jul 2020 16:56:30 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        kvm <kvm@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] kvm: x86: rewrite kvm_spec_ctrl_valid_bits

On Tue, 7 Jul 2020 at 16:15, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Aren't you supposed to be on vacation? :-)

A long vacation, enjoy!

>
> On Tue, Jul 07, 2020 at 10:04:22AM +0200, Paolo Bonzini wrote:
> > On 07/07/20 08:11, Sean Christopherson wrote:
> > > One oddity with this whole thing is that by passing through the MSR, KVM is
> > > allowing the guest to write bits it doesn't know about, which is definitely
> > > not normal.  It also means the guest could write bits that the host VMM
> > > can't.
> >
> > That's true.  However, the main purpose of the kvm_spec_ctrl_valid_bits
> > check is to ensure that host-initiated writes are valid; this way, you
> > don't get a #GP on the next vmentry's WRMSR to MSR_IA32_SPEC_CTRL.
> > Checking the guest CPUID bit is not even necessary.
>
> Right, what I'm saying is that rather than try and decipher specs to
> determine what bits are supported, just throw the value at hardware and
> go from there.  That's effectively what we end up doing for the guest writes
> anyways.
>
> Actually, the current behavior will break migration if there are ever legal
> bits that KVM doesn't recognize, e.g. guest writes a value that KVM doesn't
> allow and then migration fails when the destination tries to stuff the value
> into KVM.