linux-kernel - Re: [CRIU] x86: Hardware breakpoints are not always triggered

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Message-Id: <D2A0BC47-AA2D-4697-8F0C-F9176874DE25@gmail.com>
Date:	Wed, 3 Feb 2016 10:18:40 -0800
From:	Nadav Amit <nadav.amit@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Andrew Vagin <avagin@...tuozzo.com>,
	Andrey Wagin <avagin@...il.com>, kvm@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"criu@...nvz.org" <criu@...nvz.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: Re: [CRIU] x86: Hardware breakpoints are not always triggered

I think the problem lies here:

        /*
         * If the guest has used debug registers, at least dr7
         * will be disabled while returning to the host.
         * If we don't have active breakpoints in the host, we don't
         * care about the messed up debug address registers. But if
         * we have some of them active, restore the old state.
         */
        if (hw_breakpoint_active()) {
                hw_breakpoint_restore();
        }

First we need to reload the DRs after we do hw_breakpoint_restore(), no?
	vcpu->arch.switch_db_regs |= KVM_DEBUGREG_RELOAD;

Second, I am unsure whether hw_breakpoint_active() is the correct condition.

In my defense note that my tests did not his this case since I set affinity
of the VCPUs to physical cores, and prevented other processes from running.

Regards,
Nadav

Paolo Bonzini <pbonzini@...hat.com> wrote:

> 
> 
> On 29/01/2016 23:21, Andrew Vagin wrote:
>> On Thu, Jan 28, 2016 at 02:42:25PM -0800, Andrey Wagin wrote:
>>> On Thu, Jan 28, 2016 at 10:33:28PM +0100, Paolo Bonzini wrote:
>>>> On 28/01/2016 09:31, Andrey Wagin wrote:
>>>>> I tried to print drX registers after a break-point. Looks like they
>>>>> are set correctly.
>>>> 
>>>> Can you try this KVM patch?
>>> 
>>> Looks like it fixes a case when reproducers are running only in VM.
>> 
>> Actually Oleg's reproducer detects the bug with this patch when they are
>> rinning only in VM.
> 
> That's actually a good thing, because the patch was a long shot and I
> had no clue _why_ it would have fixed the bug.  Oleg's reproducer
> spanning host and a VM actually gives me an idea of what is going on,
> I'll try to reproduce this week.
> 
> Paolo
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