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Message-ID: <81c235cc-1198-9765-d1e7-a158ea63eac4@redhat.com>
Date:   Tue, 27 Sep 2022 11:32:43 +0200
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Subject: Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update
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To:     David Hildenbrand <david@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>
 <a8c40c94-771c-ca3d-ee1d-44cbed2398e8@redhat.com>
From:   Emanuele Giuseppe Esposito <eesposit@redhat.com>
In-Reply-To: <a8c40c94-771c-ca3d-ee1d-44cbed2398e8@redhat.com>
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Am 27/09/2022 um 11:22 schrieb David Hildenbrand:
> 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!

Anytime :)

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

Invalidation creates a new temporary identical memslot, I am not sure
about the rmaps. It is anyways the same code as it was done before and
if I understand correctly, a new slot is required to keep the old
intact, in case something goes wrong and we need to revert.

Thanks,
Emanuele