linux-kernel - Re: [PATCH RFC 1/3] x86/kvm/vmx: read MSR_FS

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Message-ID: <CALCETrX0CB_+QvDH3B8fYdH+ekRRX-i=jZU7zQ8qYbGzDrM-uQ@mail.gmail.com>
Date:   Fri, 2 Mar 2018 20:18:18 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH RFC 1/3] x86/kvm/vmx: read MSR_FS_BASE from current->thread

On Fri, Mar 2, 2018 at 10:55 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> vmx_save_host_state() is only called from kvm_arch_vcpu_ioctl_run() so
> the context is pretty well defined
>

True.

> and MSR_FS_BASE should always be
> equal to current->thread.fsbase.

Not true.  current->thread.fsbase is almost entirely undefined in this
context.  What you *could* do is export save_fsgs() and call it first.
When FSGSBASE support lands (which will happen eventually!), the code
in your patch will be completely wrong.

Admittedly, your patch isn't 100% bogus, but the reason is subtle and
you need lots of comments there *and* in save_fsgs().