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Date:   Mon, 26 Sep 2022 21:28:38 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Emanuele Giuseppe Esposito <eesposit@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Maxim Levitsky <mlevitsk@redhat.com>, 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
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On Mon, Sep 26, 2022, David Hildenbrand wrote:
> On 23.09.22 15:38, Emanuele Giuseppe Esposito wrote:
> > 
> > 
> > Am 23/09/2022 um 15:21 schrieb David Hildenbrand:
> > > On 23.09.22 15:10, Emanuele Giuseppe Esposito wrote:
> > > > 
> > > > 
> > > > Am 19/09/2022 um 19:30 schrieb David Hildenbrand:
> > > > > 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

...

> As Sean said "This is an awful lot of a complexity to take on for something
> that appears to be solvable in userspace."

And if the userspace solution is unpalatable for whatever reason, I'd like to
understand exactly what KVM behavior is problematic for userspace.  E.g. the
above RHBZ bug should no longer be an issue as the buggy commit has since been
reverted.

If the issue is KVM doing something nonsensical on a code fetch to MMIO, then I'd
much rather fix _that_ bug and improve KVM's user exit ABI to let userspace handle
the race _if_ userspace chooses not to pause vCPUs.