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

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Message-ID: <5f0345d2-d4d1-f4fe-86ba-6e22561cb6bd@redhat.com>
Date:   Sun, 18 Sep 2022 18:13:31 +0200
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     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>,
        David Hildenbrand <david@...hat.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



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)."

Probably @Paolo and @Maxim can add more to this.

Thank you,
Emanuele