linux-kernel - Re: [PATCH 02/16] KVM: x86/mmu: Introduce a slot flag to zap only slot leafs on slot deletion

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Message-ID: <fa88344c-cf0b-478f-9713-906aeb616da7@redhat.com>
Date: Fri, 17 May 2024 17:27:32 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Kai Huang <kai.huang@...el.com>,
 "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
 "sagis@...gle.com" <sagis@...gle.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Yan Y Zhao <yan.y.zhao@...el.com>, Erdem Aktas <erdemaktas@...gle.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH 02/16] KVM: x86/mmu: Introduce a slot flag to zap only
 slot leafs on slot deletion

On 5/15/24 22:05, Sean Christopherson wrote:
>> Again thinking of the userspace memory analogy... Aren't there some VMs where
>> the fast zap is faster? Like if you have guest with a small memslot that gets
>> deleted all the time, you could want it to be zapped specifically. But for the
>> giant memslot next to it, you might want to do the fast zap all thing.
>
> Yes.  But...

On the other hand, tearing down a giant memslot isn't really common. 
The main occurrence of memslots going away is 1) BIOS fiddling with low 
memory permissions;  2) PCI BARs.  The former is only happening at boot 
and with small memslots, the latter can in principle involve large
memslots but... just don't do it.

Paolo