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Date: Fri, 9 Sep 2022 14:30:42 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Emanuele Giuseppe Esposito <eesposit@...hat.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.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>,
David Hildenbrand <david@...hat.com>,
Maxim Levitsky <mlevitsk@...hat.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates
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?
> 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.
And if the issue is related to KVM disallowing the toggling of read-only (can't see
the bug), we can likely solve that without needing a new ioctl() that allows
userspace to batch an arbitrary number of updates.
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