linux-kernel - Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

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Date:   Tue, 27 Sep 2022 09:38:46 +0200
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        David Hildenbrand <david@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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



Am 26/09/2022 um 23:28 schrieb Sean Christopherson:
> On Mon, Sep 26, 2022, David Hildenbrand wrote:
>> On 23.09.22 15:38, Emanuele Giuseppe Esposito wrote:
>>>
>>>
>>> Am 23/09/2022 um 15:21 schrieb David Hildenbrand:
>>>> On 23.09.22 15:10, Emanuele Giuseppe Esposito wrote:
>>>>>
>>>>>
>>>>> Am 19/09/2022 um 19:30 schrieb David Hildenbrand:
>>>>>> On 19.09.22 09:53, David Hildenbrand wrote:
>>>>>>> On 18.09.22 18:13, Emanuele Giuseppe Esposito wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> Am 09/09/2022 um 16:30 schrieb Sean Christopherson:
>>>>>>>>> On Fri, Sep 09, 2022, Emanuele Giuseppe Esposito wrote:
>>>>>>>>>> KVM is currently capable of receiving a single memslot update
>>>>>>>>>> through
>>>>>>>>>> the KVM_SET_USER_MEMORY_REGION ioctl.
>>>>>>>>>> The problem arises when we want to atomically perform multiple
>>>>>>>>>> updates,
>>>>>>>>>> so that readers of memslot active list avoid seeing incomplete
>>>>>>>>>> states.
>>>>>>>>>>
>>>>>>>>>> For example, in RHBZ
>>>>>>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1979276
> 
> ...
> 
>> 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.

> 
> 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.

To be more precise, what I did mostly follows what Peter explained in
comment 19 : https://bugzilla.redhat.com/show_bug.cgi?id=1979276#c19