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From: Andy Lutomirski <luto@kernel.org>
Date: Fri, 2 Mar 2018 20:18:18 +0000
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Subject: Re: [PATCH RFC 1/3] x86/kvm/vmx: read MSR_FS_BASE from current->thread
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm list <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>
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On Fri, Mar 2, 2018 at 10:55 AM, Vitaly Kuznetsov <vkuznets@redhat.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().