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Message-ID: <a8c40c94-771c-ca3d-ee1d-44cbed2398e8@redhat.com>
Date:   Tue, 27 Sep 2022 11:22:19 +0200
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Subject: Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update
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To:     Emanuele Giuseppe Esposito <eesposit@redhat.com>,
        kvm@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        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>,
        Maxim Levitsky <mlevitsk@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
References: <20220909104506.738478-1-eesposit@redhat.com>
 <20220909104506.738478-10-eesposit@redhat.com>
 <cde8be9d-64c0-80e5-7663-4302d075dcbc@redhat.com>
 <07014070-5186-ca95-7028-82f77612dedd@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <07014070-5186-ca95-7028-82f77612dedd@redhat.com>
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On 27.09.22 10:35, Emanuele Giuseppe Esposito wrote:
> 
> 
> Am 27/09/2022 um 09:46 schrieb David Hildenbrand:
>> On 09.09.22 12:45, Emanuele Giuseppe Esposito wrote:
>>> When kvm_vm_ioctl_set_memory_region_list() is invoked, we need
>>> to make sure that all memslots are updated in the inactive list
>>> and then swap (preferreably only once) the lists, so that all
>>> changes are visible immediately.
>>>
>>> The only issue is that DELETE and MOVE need to perform 2 swaps:
>>> firstly replace old memslot with invalid, and then remove invalid.
>>>
>>
>> I'm curious, how would a resize (grow/shrink) or a split be handled?
>>
> 
> There are only 4 operations possible in KVM: KVM_MR_{DELETE, MOVE,
> CREATE, FLAGS_ONLY}.
> 
> A resize should be implemented in QEMU as DELETE+CREATE.
> 
> Therefore a resize on memslot X will be implemented as:
> First pass on the userspace operations:
> 	invalidate memslot X;
> 	swap_memslot_list(); // NOW it is visible to the guest
> 
> What guest sees: memslot X is invalid, so MMU keeps retrying the page fault
> 
> Second pass:
> 	create new memslot X
> 	delete old memslot X

Thanks a lot for the very nice explanation!

Does the invalidation already free up memslot metadata (especially the 
rmaps) or will we end up temporarily allocating twice the memslot metadata?

-- 
Thanks,

David / dhildenb