From: David Hildenbrand <david@redhat.com>
To: Emanuele Giuseppe Esposito <eesposit@redhat.com>,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, 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>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
linux-kernel@vger.kernel.org, Like Xu <like.xu.linux@gmail.com>
Subject: Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates
Date: Mon, 19 Sep 2022 19:30:31 +0200 [thread overview]
Message-ID: <dfcbdf1d-b078-ec6c-7706-6af578f79ec2@redhat.com> (raw)
In-Reply-To: <37b3162e-7b3a-919f-80e2-f96eca7d4b4c@redhat.com>
On 19.09.22 09:53, David Hildenbrand wrote:
> On 18.09.22 18:13, Emanuele Giuseppe Esposito wrote:
>>
>>
>> Am 09/09/2022 um 16:30 schrieb Sean Christopherson:
>>> On Fri, Sep 09, 2022, Emanuele Giuseppe Esposito wrote:
>>>> KVM is currently capable of receiving a single memslot update through
>>>> the KVM_SET_USER_MEMORY_REGION ioctl.
>>>> The problem arises when we want to atomically perform multiple updates,
>>>> so that readers of memslot active list avoid seeing incomplete states.
>>>>
>>>> For example, in RHBZ https://bugzilla.redhat.com/show_bug.cgi?id=1979276
>>>
>>> I don't have access. Can you provide a TL;DR?
>>
>> You should be able to have access to it now.
>>
>>>
>>>> we see how non atomic updates cause boot failure, because vcpus
>>>> will se a partial update (old memslot delete, new one not yet created)
>>>> and will crash.
>>>
>>> Why not simply pause vCPUs in this scenario? This is an awful lot of a complexity
>>> to take on for something that appears to be solvable in userspace.
>>>
>>
>> I think it is not that easy to solve in userspace: see
>> https://lore.kernel.org/qemu-devel/20200312161217.3590-1-david@redhat.com/
>>
>>
>> "Using pause_all_vcpus()/resume_all_vcpus() is not possible, as it will
>> temporarily drop the BQL - something most callers can't handle (esp.
>> when called from vcpu context e.g., in virtio code)."
>
> Can you please comment on the bigger picture? The patch from me works
> around *exactly that*, and for that reason, contains that comment.
>
FWIW, I hacked up my RFC to perform atomic updates on any memslot
transactions (not just resizes) where ranges do add overlap with ranges
to remove.
https://github.com/davidhildenbrand/qemu/tree/memslot
I only performed simple boot check under x86-64 (where I can see region
resizes) and some make checks -- pretty sure it has some rough edges;
but should indicate what's possible and what the possible price might
be. [one could wire up a new KVM ioctl and call it conditionally on
support if really required]
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2022-09-19 17:30 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-09 10:44 Emanuele Giuseppe Esposito
2022-09-09 10:44 ` [RFC PATCH 1/9] kvm_main.c: move slot check in kvm_set_memory_region Emanuele Giuseppe Esposito
2022-09-28 16:41 ` Paolo Bonzini
2022-09-09 10:44 ` [RFC PATCH 2/9] kvm.h: introduce KVM_SET_USER_MEMORY_REGION_LIST ioctl Emanuele Giuseppe Esposito
2022-09-28 16:42 ` Paolo Bonzini
2022-09-09 10:45 ` [RFC PATCH 3/9] kvm_main.c: introduce kvm_internal_memory_region_list Emanuele Giuseppe Esposito
2022-09-28 16:48 ` Paolo Bonzini
2022-09-09 10:45 ` [RFC PATCH 4/9] kvm_main.c: split logic in kvm_set_memslots Emanuele Giuseppe Esposito
2022-09-28 17:04 ` Paolo Bonzini
2022-09-09 10:45 ` [RFC PATCH 5/9] kvm_main.c: split __kvm_set_memory_region logic in kvm_check_mem and kvm_prepare_batch Emanuele Giuseppe Esposito
2022-09-13 2:56 ` Yang, Weijiang
2022-09-18 16:22 ` Emanuele Giuseppe Esposito
2022-09-28 17:11 ` Paolo Bonzini
2022-09-09 10:45 ` [RFC PATCH 6/9] kvm_main.c: simplify change-specific callbacks Emanuele Giuseppe Esposito
2022-09-09 10:45 ` [RFC PATCH 7/9] kvm_main.c: duplicate invalid memslot also in inactive list Emanuele Giuseppe Esposito
2022-09-28 17:18 ` Paolo Bonzini
2022-09-09 10:45 ` [RFC PATCH 8/9] kvm_main.c: find memslots from the inactive memslot list Emanuele Giuseppe Esposito
2022-09-09 10:45 ` [RFC PATCH 9/9] kvm_main.c: handle atomic memslot update Emanuele Giuseppe Esposito
2022-09-13 2:30 ` Yang, Weijiang
2022-09-18 16:18 ` Emanuele Giuseppe Esposito
2022-09-27 7:46 ` David Hildenbrand
2022-09-27 8:35 ` Emanuele Giuseppe Esposito
2022-09-27 9:22 ` David Hildenbrand
2022-09-27 9:32 ` Emanuele Giuseppe Esposito
2022-09-27 14:52 ` David Hildenbrand
2022-09-28 17:29 ` Paolo Bonzini
2022-09-09 14:30 ` [RFC PATCH 0/9] kvm: implement atomic memslot updates Sean Christopherson
2022-09-18 16:13 ` Emanuele Giuseppe Esposito
2022-09-19 7:38 ` Like Xu
2022-09-19 7:53 ` David Hildenbrand
2022-09-19 17:30 ` David Hildenbrand [this message]
2022-09-23 13:10 ` Emanuele Giuseppe Esposito
2022-09-23 13:21 ` David Hildenbrand
2022-09-23 13:38 ` Emanuele Giuseppe Esposito
2022-09-26 9:03 ` David Hildenbrand
2022-09-26 21:28 ` Sean Christopherson
2022-09-27 7:38 ` Emanuele Giuseppe Esposito
2022-09-27 15:58 ` Sean Christopherson
2022-09-28 9:11 ` Emanuele Giuseppe Esposito
2022-09-28 11:14 ` Maxim Levitsky
2022-09-28 12:52 ` David Hildenbrand
2022-09-28 15:07 ` Paolo Bonzini
2022-09-28 15:33 ` David Hildenbrand
2022-09-28 15:58 ` Sean Christopherson
2022-09-28 16:38 ` Paolo Bonzini
2022-09-28 20:41 ` Sean Christopherson
2022-09-29 8:05 ` Emanuele Giuseppe Esposito
2022-09-29 8:24 ` David Hildenbrand
2022-09-29 15:18 ` Sean Christopherson
2022-09-29 15:41 ` Paolo Bonzini
2022-09-29 15:28 ` Paolo Bonzini
2022-09-29 15:40 ` Maxim Levitsky
2022-09-29 16:00 ` David Hildenbrand
2022-09-29 21:39 ` Sean Christopherson
2022-10-13 7:43 ` Emanuele Giuseppe Esposito
2022-10-13 8:44 ` David Hildenbrand
2022-10-13 11:12 ` Emanuele Giuseppe Esposito
2022-10-13 14:45 ` David Hildenbrand
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