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Subject: Re: [PATCH] KVM: x86/mmu: Add capability to zap only sptes for the
 affected memslot
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Xiong Zhang <xiong.y.zhang@intel.com>,
        Wayne Boyer <wayne.boyer@intel.com>,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>
References: <20200703025047.13987-1-sean.j.christopherson@intel.com>
 <51637a13-f23b-8b76-c93a-76346b4cc982@redhat.com>
 <20200709211253.GW24919@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <49c7907a-3ab4-b5db-ccb4-190b990c8be3@redhat.com>
Date:   Fri, 10 Jul 2020 00:18:17 +0200
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On 09/07/20 23:12, Sean Christopherson wrote:
>> It's bad that we have no clue what's causing the bad behavior, but I
>> don't think it's wise to have a bug that is known to happen when you
>> enable the capability. :/

(Note that this wasn't a NACK, though subtly so).

> I don't necessarily disagree, but at the same time it's entirely possible
> it's a Qemu bug.

No, it cannot be.  QEMU is not doing anything but
KVM_SET_USER_MEMORY_REGION, and it's doing that synchronously with
writes to the PCI configuration space BARs.

> Even if this is a kernel bug, I'm fairly confident at this point that it's
> not a KVM bug.  Or rather, if it's a KVM "bug", then there's a fundamental
> dependency in memslot management that needs to be rooted out and documented.

Heh, here my surmise is that  it cannot be anything but a KVM bug,
because  Memslots are not used by anything outside KVM...  But maybe I'm
missing something.

> And we're kind of in a catch-22; it'll be extremely difficult to narrow down
> exactly who is breaking what without being able to easily test the optimized
> zapping with other VMMs and/or setups.

I agree with this, and we could have a config symbol that depends on
BROKEN and enables it unconditionally.  However a capability is the
wrong tool.

Paolo