From: Jan Kiszka <jan.kiszka@siemens.com>
To: Maxim Levitsky <mlevitsk@redhat.com>,
Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, Vitaly Kuznetsov <vkuznets@redhat.com>,
linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Wanpeng Li <wanpengli@tencent.com>,
Kieran Bingham <kbingham@kernel.org>,
Jessica Yu <jeyu@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, Joerg Roedel <joro@8bytes.org>,
Jim Mattson <jmattson@google.com>, Borislav Petkov <bp@alien8.de>,
Stefano Garzarella <sgarzare@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping
Date: Tue, 16 Mar 2021 12:27:18 +0100 [thread overview]
Message-ID: <ca41fe98-0e5d-3b4c-8ed8-bdd7cd5bc60f@siemens.com> (raw)
In-Reply-To: <bede3450413a7c5e7e55b19a47c8f079edaa55a2.camel@redhat.com>
On 16.03.21 11:59, Maxim Levitsky wrote:
> On Tue, 2021-03-16 at 10:16 +0100, Jan Kiszka wrote:
>> On 16.03.21 00:37, Sean Christopherson wrote:
>>> On Tue, Mar 16, 2021, Maxim Levitsky wrote:
>>>> This change greatly helps with two issues:
>>>>
>>>> * Resuming from a breakpoint is much more reliable.
>>>>
>>>> When resuming execution from a breakpoint, with interrupts enabled, more often
>>>> than not, KVM would inject an interrupt and make the CPU jump immediately to
>>>> the interrupt handler and eventually return to the breakpoint, to trigger it
>>>> again.
>>>>
>>>> From the user point of view it looks like the CPU never executed a
>>>> single instruction and in some cases that can even prevent forward progress,
>>>> for example, when the breakpoint is placed by an automated script
>>>> (e.g lx-symbols), which does something in response to the breakpoint and then
>>>> continues the guest automatically.
>>>> If the script execution takes enough time for another interrupt to arrive,
>>>> the guest will be stuck on the same breakpoint RIP forever.
>>>>
>>>> * Normal single stepping is much more predictable, since it won't land the
>>>> debugger into an interrupt handler, so it is much more usable.
>>>>
>>>> (If entry to an interrupt handler is desired, the user can still place a
>>>> breakpoint at it and resume the guest, which won't activate this workaround
>>>> and let the gdb still stop at the interrupt handler)
>>>>
>>>> Since this change is only active when guest is debugged, it won't affect
>>>> KVM running normal 'production' VMs.
>>>>
>>>>
>>>> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
>>>> Tested-by: Stefano Garzarella <sgarzare@redhat.com>
>>>> ---
>>>> arch/x86/kvm/x86.c | 6 ++++++
>>>> 1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index a9d95f90a0487..b75d990fcf12b 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -8458,6 +8458,12 @@ static void inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit
>>>> can_inject = false;
>>>> }
>>>>
>>>> + /*
>>>> + * Don't inject interrupts while single stepping to make guest debug easier
>>>> + */
>>>> + if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
>>>> + return;
>>>
>>> Is this something userspace can deal with? E.g. disable IRQs and/or set NMI
>>> blocking at the start of single-stepping, unwind at the end? Deviating this far
>>> from architectural behavior will end in tears at some point.
>>>
>>
>> Does this happen to address this suspicious workaround in the kernel?
>>
>> /*
>> * The kernel doesn't use TF single-step outside of:
>> *
>> * - Kprobes, consumed through kprobe_debug_handler()
>> * - KGDB, consumed through notify_debug()
>> *
>> * So if we get here with DR_STEP set, something is wonky.
>> *
>> * A known way to trigger this is through QEMU's GDB stub,
>> * which leaks #DB into the guest and causes IST recursion.
>> */
>> if (WARN_ON_ONCE(dr6 & DR_STEP))
>> regs->flags &= ~X86_EFLAGS_TF;
>>
>> (arch/x86/kernel/traps.c, exc_debug_kernel)
>>
>> I wonder why this got merged while no one fixed QEMU/KVM, for years? Oh,
>> yeah, question to myself as well, dancing around broken guest debugging
>> for a long time while trying to fix other issues...
>
> To be honest I didn't see that warning even once, but I can imagine KVM
> leaking #DB due to bugs in that code. That area historically didn't receive
> much attention since it can only be triggered by
> KVM_GET/SET_GUEST_DEBUG which isn't used in production.
I've triggered it recently while debugging a guest, that's why I got
aware of the code path. Long ago, all this used to work (soft BPs,
single-stepping etc.)
>
> The only issue that I on the other hand did
> see which is mostly gdb fault is that it fails to remove a software breakpoint
> when resuming over it, if that breakpoint's python handler messes up
> with gdb's symbols, which is what lx-symbols does.
>
> And that despite the fact that lx-symbol doesn't mess with the object
> (that is the kernel) where the breakpoint is defined.
>
> Just adding/removing one symbol file is enough to trigger this issue.
>
> Since lx-symbols already works this around when it reloads all symbols,
> I extended that workaround to happen also when loading/unloading
> only a single symbol file.
You have no issue with interactive debugging when NOT using gdb scripts
/ lx-symbol?
Jan
--
Siemens AG, T RDA IOT
Corporate Competence Center Embedded Linux
next prev parent reply other threads:[~2021-03-16 11:28 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 22:10 [PATCH 0/3] KVM: my debug patch queue Maxim Levitsky
2021-03-15 22:10 ` [PATCH 1/3] scripts/gdb: rework lx-symbols gdb script Maxim Levitsky
2021-03-16 13:38 ` Jan Kiszka
2021-03-16 14:12 ` Maxim Levitsky
2021-03-15 22:10 ` [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping Maxim Levitsky
2021-03-15 23:37 ` Sean Christopherson
2021-03-16 9:16 ` Jan Kiszka
2021-03-16 10:59 ` Maxim Levitsky
2021-03-16 11:27 ` Jan Kiszka [this message]
2021-03-16 12:34 ` Maxim Levitsky
2021-03-16 13:46 ` Jan Kiszka
2021-03-16 14:34 ` Maxim Levitsky
2021-03-16 15:31 ` Jan Kiszka
[not found] ` <e2cd978e357155dbab21a523bb8981973bd10da7.camel@redhat.com>
2021-03-16 15:56 ` Jan Kiszka
2021-03-16 16:50 ` Sean Christopherson
2021-03-16 17:01 ` Jan Kiszka
2021-03-16 17:26 ` Sean Christopherson
2021-03-17 9:20 ` Jan Kiszka
2021-03-16 18:02 ` Maxim Levitsky
2021-03-18 16:02 ` Jan Kiszka
2021-03-18 12:21 ` Jan Kiszka
2021-03-16 10:55 ` Maxim Levitsky
2021-03-15 22:10 ` [PATCH 3/3] KVM: SVM: allow to intercept all exceptions for debug Maxim Levitsky
2021-03-16 8:32 ` Joerg Roedel
2021-03-16 10:51 ` Maxim Levitsky
2021-03-18 9:19 ` Joerg Roedel
2021-03-18 9:24 ` Maxim Levitsky
2021-03-18 15:47 ` Joerg Roedel
2021-03-18 16:35 ` Sean Christopherson
2021-03-18 16:41 ` Maxim Levitsky
2021-03-18 17:22 ` Sean Christopherson
2021-03-16 8:34 ` Borislav Petkov
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