linux-kernel - Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update

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Date:   Tue, 27 Sep 2022 11:22:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        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>,
        Maxim Levitsky <mlevitsk@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update

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