Date:   Wed, 28 Sep 2022 17:33:53 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Like Xu <like.xu.linux@...il.com>
Subject: Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

On 28.09.22 17:07, Paolo Bonzini wrote:
> On 9/27/22 17:58, Sean Christopherson wrote:
>> On Tue, Sep 27, 2022, Emanuele Giuseppe Esposito wrote:
>>>
>>> Am 26/09/2022 um 23:28 schrieb Sean Christopherson:
>>>> On Mon, Sep 26, 2022, David Hildenbrand wrote:
>>>>> As Sean said "This is an awful lot of a complexity to take on for something
>>>>> that appears to be solvable in userspace."
>>>>
>>>> And if the userspace solution is unpalatable for whatever reason, I'd like to
>>>> understand exactly what KVM behavior is problematic for userspace.  E.g. the
>>>> above RHBZ bug should no longer be an issue as the buggy commit has since been
>>>> reverted.
>>>
>>> It still is because I can reproduce the bug, as also pointed out in
>>> multiple comments below.
>>
>> You can reproduce _a_ bug, but it's obviously not the original bug, because the
>> last comment says:
>>
>>     Second, indeed the patch was reverted and somehow accepted without generating
>>     too much noise:
>>
>>     ...
>>
>>     The underlying issue of course as we both know is still there.
>>
>>     You might have luck reproducing it with this bug
>>
>>     https://bugzilla.redhat.com/show_bug.cgi?id=1855298
>>
>>     But for me it looks like it is 'working' as well, so you might have
>>     to write a unit test to trigger the issue.
>>
>>>> If the issue is KVM doing something nonsensical on a code fetch to MMIO, then I'd
>>>> much rather fix _that_ bug and improve KVM's user exit ABI to let userspace handle
>>>> the race _if_ userspace chooses not to pause vCPUs.
>>>>
>>>
>>> Also on the BZ they all seem (Paolo included) to agree that the issue is
>>> non-atomic memslots update.
>>
>> Yes, non-atomic memslot likely results in the guest fetching from a GPA without a
>> memslot.  I'm asking for an explanation of exactly what happens when that occurs,
>> because it should be possible to adjust KVM and/or QEMU to play nice with the
>> fetch, e.g. to resume the guest until the new memslot is installed, in which case
>> an atomic update isn't needed.
>>
>> I assume the issue is that KVM exits with KVM_EXIT_INTERNAL_ERROR because the
>> guest is running at CPL=0, and QEMU kills the guest in response.  If that's correct,
>> then that problem can be solved by exiting to userspace with KVM_EXIT_MMIO instead
>> of KVM_EXIT_INTERNAL_ERROR so that userspace can do something sane in response to
>> the MMIO code fetch.
>>
>> I'm pretty sure this patch will Just Work for QEMU, because QEMU simply resumes
>> the vCPU if mmio.len==0.  It's a bit of a hack, but I don't think it violates KVM's
>> ABI in any way, and it can even become "official" behavior since KVM x86 doesn't
>> otherwise exit with mmio.len==0.
> 
> I think this patch is not a good idea for two reasons:
> 
> 1) we don't know how userspace behaves if mmio.len is zero.  It is of
> course reasonable to do nothing, but an assertion failure is also a
> valid behavior
> 
> 2) more important, there is no way to distinguish a failure due to the
> guest going in the weeds (and then KVM_EXIT_INTERNAL_ERROR is fine) from
> one due to the KVM_SET_USER_MEMORY_REGION race condition.  So this will
> cause a guest that correctly caused an internal error to loop forever.
> 
> While the former could be handled in a "wait and see" manner, the latter
> in particular is part of the KVM_RUN contract.  Of course it is possible
> for a guest to just loop forever, but in general all of KVM, QEMU and
> upper userspace layers want a crashed guest to be detected and stopped
> forever.
> 
> Yes, QEMU could loop only if memslot updates are in progress, but
> honestly all the alternatives I have seen to atomic memslot updates are
> really *awful*.  David's patches even invent a new kind of mutex for
> which I have absolutely no idea what kind of deadlocks one should worry
> about and why they should not exist; QEMU's locking is already pretty
> crappy, it's certainly not on my wishlist to make it worse!

Just to comment on that (I'm happy as long as this gets fixed), a simple 
mutex with trylock should get the thing done as well -- kicking the VCPU 
if the trylock fails. But I did not look further into locking alternatives.

-- 
Thanks,

David / dhildenb

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