linux-kernel - Re: [patch V5 08/15] x86/entry: Move user return notifier out of loop

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Date:   Thu, 23 Jul 2020 16:41:11 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        linux-arch@...r.kernel.org, Will Deacon <will@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Keno Fischer <keno@...iacomputing.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: [patch V5 08/15] x86/entry: Move user return notifier out of loop

On Thu, Jul 23, 2020 at 12:00:02AM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Guests and user space share certain MSRs. KVM sets these MSRs to guest
> values once and does not set them back to user space values on every VM
> exit to spare the costly MSR operations.
> 
> User return notifiers ensure that these MSRs are set back to the correct
> values before returning to user space in exit_to_usermode_loop().
> 
> There is no reason to evaluate the TIF flag indicating that user return
> notifiers need to be invoked in the loop. The important point is that they
> are invoked before returning to user space.
> 
> Move the invocation out of the loop into the section which does the last
> preperatory steps before returning to user space. That section is not
> preemptible and runs with interrupts disabled until the actual return.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-and-tested-by: Sean Christopherson <sean.j.christopherson@...el.com>