Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <07014070-5186-ca95-7028-82f77612dedd@redhat.com>
Date:   Tue, 27 Sep 2022 10:35:42 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Subject: Re: [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update
Content-Language: en-US
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>
From:   Emanuele Giuseppe Esposito <eesposit@redhat.com>
In-Reply-To: <cde8be9d-64c0-80e5-7663-4302d075dcbc@redhat.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk


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


What guest sees: memslot X is invalid, so MMU keeps retrying the page fault

Third pass:
	swap() // 1 for each address space affected

What guest sees: new memslot replacing the old one


A split is DELETE+CREATE+CREATE, so it's the same:

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:
	delete old memslot X
	create new memslot X1
	create new memslot X2

What guest sees: memslot X is invalid, so MMU keeps retrying the page fault

Third pass:
	swap() // 1 for each address space affected

What guest sees: two new memslots replacing the old one