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: <175989e7-2275-4775-9ad8-65c4134184dd@intel.com>
Date: Thu, 16 May 2024 11:06:35 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Rick P Edgecombe
	<rick.p.edgecombe@...el.com>
CC: "dmatlack@...gle.com" <dmatlack@...gle.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>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>
Subject: Re: [PATCH 02/16] KVM: x86/mmu: Introduce a slot flag to zap only
 slot leafs on slot deletion


>>
>> You had said up the thread, why not opt all non-normal VMs into the new
>> behavior. It will work great for TDX. But why do SEV and others want this
>> automatically?
> 
> Because I want flexibility in KVM, i.e. I want to take the opportunity to try and
> break away from KVM's godawful ABI.  It might be a pipe dream, as keying off the
> VM type obviously has similar risks to giving userspace a memslot flag.  The one
> sliver of hope is that the VM types really are quite new (though less so for SEV
> and SEV-ES), whereas a memslot flag would be easily applied to existing VMs.

Btw, does the "zap-leaf-only" approach always have better performance, 
assuming we have to hold MMU write lock for that?

Consider a huge memslot being deleted/moved.

If we can always have a better performance for "zap-leaf-only", then 
instead of letting userspace to opt-in this feature, we perhaps can do 
the opposite:

We always do the "zap-leaf-only" in KVM, but add a quirk for the VMs 
that userspace know can have such bug and apply this quirk.

But again, I think it's just too overkill for TDX.  We can just set the 
ZAP_LEAF_ONLY flag for the slot when it is created in KVM.