linux-kernel - Re: [PATCH 0/4] KVM: API to block and resume all running vcpus in a vm

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Date:   Tue, 25 Oct 2022 11:33:29 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        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-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] KVM: API to block and resume all running vcpus in a
 vm

On 10/25/22 00:45, Sean Christopherson wrote:
>> Yes that helps and should be part of the cover letter for the next iterations.
> But that doesn't explain why KVM needs to get involved, it only explains why QEMU
> can't use its existing pause_all_vcpus().  I do not understand why this is a
> problem QEMU needs KVM's help to solve.

I agree that it's not KVM's problem that QEMU cannot use 
pause_all_vcpus().  Having a ioctl in KVM, rather than coding the same 
in QEMU, is *mostly* a matter of programmer and computer efficiency 
because the code is pretty simple.

That said, I believe the limited memslot API makes it more than just a 
QEMU problem.  Because KVM_GET_DIRTY_LOG cannot be combined atomically 
with KVM_SET_USER_MEMORY_REGION(MR_DELETE), any VMM that uses dirty-log 
regions while the VM is running is liable to losing the dirty status of 
some pages.  That's also a reason to provide this API in KVM.

Paolo