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Date:   Mon, 19 Sep 2022 19:30:31 +0200
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From:   David Hildenbrand <david@redhat.com>
To:     Emanuele Giuseppe Esposito <eesposit@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
Cc:     kvm@vger.kernel.org, Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Like Xu <like.xu.linux@gmail.com>
References: <20220909104506.738478-1-eesposit@redhat.com>
 <YxtOEgJhe4EcAJsE@google.com>
 <5f0345d2-d4d1-f4fe-86ba-6e22561cb6bd@redhat.com>
 <37b3162e-7b3a-919f-80e2-f96eca7d4b4c@redhat.com>
Organization: Red Hat
Subject: Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates
In-Reply-To: <37b3162e-7b3a-919f-80e2-f96eca7d4b4c@redhat.com>
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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
>>>
>>> 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.
> 

FWIW, I hacked up my RFC to perform atomic updates on any memslot 
transactions (not just resizes) where ranges do add overlap with ranges 
to remove.

https://github.com/davidhildenbrand/qemu/tree/memslot


I only performed simple boot check under x86-64 (where I can see region 
resizes) and some make checks -- pretty sure it has some rough edges; 
but should indicate what's possible and what the possible price might 
be. [one could wire up a new KVM ioctl and call it conditionally on 
support if really required]


-- 
Thanks,

David / dhildenb