From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory
Date: Wed, 29 Nov 2017 14:45:21 -0800
Message-ID: <20171129224521.GF3070@tassilo.jf.intel.com>
References: <1511935673-7371-1-git-send-email-wanpeng.li@hotmail.com>
 <87fu8xj69o.fsf@linux.intel.com>
 <5045320c-3036-4e08-140e-4bb5cf78be24@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Jim Mattson <jmattson@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mga02.intel.com ([134.134.136.20]:34278 "EHLO mga02.intel.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751771AbdK2WpX (ORCPT <rfc822;kvm@vger.kernel.org>);
        Wed, 29 Nov 2017 17:45:23 -0500
Content-Disposition: inline
In-Reply-To: <5045320c-3036-4e08-140e-4bb5cf78be24@redhat.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On Wed, Nov 29, 2017 at 11:26:30PM +0100, Paolo Bonzini wrote:
> On 29/11/2017 19:20, Andi Kleen wrote:
> > But I haven't looked too closely, but I suspect you'll clobber global
> > kernel debugger state this way.
> 
> I checked all callers of update_debugctlmsr, and couldn't find any that
> could run asynchronously while KVM is caching the value.  For example
> __switch_to_xtra would always run before the sched_in notifier.

True. It would only be a problem if the debugger supported branch stepping
or LBRs, which it doesn't seem to currently.

-Andi