From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH] xen/arm: Init traps very early Date: Wed, 28 May 2014 15:16:13 +0100 Message-ID: <5385EFAD.90101@citrix.com> References: <1401283983-19843-1-git-send-email-julien.grall@linaro.org> <5385E956.1040104@citrix.com> <5385EB80.5050905@linaro.org> <5385ECFD.5080702@citrix.com> <5385EE2B.6030109@linaro.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1WpeeR-0008A8-Jz for xen-devel@lists.xenproject.org; Wed, 28 May 2014 14:16:19 +0000 In-Reply-To: <5385EE2B.6030109@linaro.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Julien Grall Cc: naresh.bhat@linaro.org, xen-devel@lists.xenproject.org, stefano.stabellini@citrix.com, ian.campbell@citrix.com, tim@xen.org List-Id: xen-devel@lists.xenproject.org On 28/05/14 15:09, Julien Grall wrote: > On 05/28/2014 03:04 PM, Andrew Cooper wrote: >> On 28/05/14 14:58, Julien Grall wrote: >>> Hi Andrew, >>> >>> On 05/28/2014 02:49 PM, Andrew Cooper wrote: >>>> On 28/05/14 14:33, Julien Grall wrote: >>>>> The function init_traps setups the handler taken when Xen hits a BUG_ON/ASSERT. >>>>> >>>>> If an error happen before init_traps is called, we loose the backtrace. >>>>> >>>>> As the function doesn't require any specific setup, we can call it just >>>>> after Xen has jumped in C code. >>>>> >>>>> Signed-off-by: Julien Grall >>>>> --- >>>>> xen/arch/arm/setup.c | 3 +-- >>>>> 1 file changed, 1 insertion(+), 2 deletions(-) >>>>> >>>>> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c >>>>> index b9ce7a9..5bf8686 100644 >>>>> --- a/xen/arch/arm/setup.c >>>>> +++ b/xen/arch/arm/setup.c >>>>> @@ -666,6 +666,7 @@ void __init start_xen(unsigned long boot_phys_offset, >>>>> struct domain *dom0; >>>>> >>>>> setup_cache(); >>>>> + init_traps(); >>>> Having recently redone this in x86, it must be after >>>> set_processor_id(0), set_current() for debug sanity, and after >>>> percpu_init_areas() for future proofing. >>> Even though it doesn't harm to call before (the stack will be >>> corrupted), I agree to call init_traps after set_processor_id(0), and >>> therefore percpu_init_areas. >>> >>> But we don't need to have set_current correctly set up. The trap entries >>> won't save anything about the guest if the exception are taken from HYP >>> mode. >>> >> set_current((struct vcpu *)0xfffff000); /* debug sanity */ >> >> This is designed to force a pagefault if a trap handler tries to use >> current before current is a valid vcpu. It is a debugging measure >> rather than a functional one, and does pick up issues. > At this stage if we hit a trap it's mostly for a data abort, alignment > issue, undefined instruction. *none* of them if using current. > > I prefer to move init_traps earlier and get useful call stack 95% of the > time, rather than loosing it for the sack of debug sanity. > > It's very painful to debug when the ASSERT is in the header and can be > called on multiple place. > > Regards, > You are guarding against changes in the future, not the current behaviour. Having correctly set up per_cpu areas, processor IDs and 'current' are crucial before the trap handlers can be safely run in general. In the end, set_processor_id(), set_current() and percpu_init_areas() are trivial operations which will not fault. ~Andrew