From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH] x86/traps: Avoid the #GP slowpath for guest #DB exceptions Date: Fri, 9 Jan 2015 17:09:41 +0000 Message-ID: <54B00B55.9020102@citrix.com> References: <1420723539-16288-1-git-send-email-andrew.cooper3@citrix.com> <54AE9B770200007800052AE1@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54AE9B770200007800052AE1@mail.emea.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: Tim Deegan , Keir Fraser , Xen-devel List-Id: xen-devel@lists.xenproject.org On 08/01/15 14:00, Jan Beulich wrote: >>>> On 08.01.15 at 14:25, wrote: >> do_debug() is capable of correctly dealing with #DB exceptions in guest >> context, and indeed needs to be as the 'icebp' instruction skips the DPL >> check anyway. > I don't follow: ICEBP doesn't check DPL, right, but what does setting > DPL to 3 buy us? Other than for INTO and INT3, we don't want to > encourage use of INT $0x01 instructions, nor am I aware of anyone > commonly using them. Yet afaict only they would be affected by your > change. Actual #DB, just like any other hardware exceptions, don't > consider DPL just like ICEBP doesn't. > > Jan > I noticed it as I was attempting to get my debugtraps test case working for PV guests, which does indeed use 'int $0x1'. But as you point out, it is only 'int $0x1' we would gain a fastpath for, which doesn't really make it worthwhile. ~Andrew