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

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Message-ID: <37b3162e-7b3a-919f-80e2-f96eca7d4b4c@redhat.com>
Date:   Mon, 19 Sep 2022 09:53:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     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
Subject: Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

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
>>
>> I don't have access.  Can you provide a TL;DR?
> 
> You should be able to have access to it now.
> 
>>
>>> we see how non atomic updates cause boot failure, because vcpus
>>> will se a partial update (old memslot delete, new one not yet created)
>>> and will crash.
>>
>> Why not simply pause vCPUs in this scenario?  This is an awful lot of a complexity
>> to take on for something that appears to be solvable in userspace.
>>
> 
> I think it is not that easy to solve in userspace: see
> https://lore.kernel.org/qemu-devel/20200312161217.3590-1-david@redhat.com/
> 
> 
> "Using pause_all_vcpus()/resume_all_vcpus() is not possible, as it will
> temporarily drop the BQL - something most callers can't handle (esp.
> when called from vcpu context e.g., in virtio code)."

Can you please comment on the bigger picture? The patch from me works 
around *exactly that*, and for that reason, contains that comment.

-- 
Thanks,

David / dhildenb