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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Date: Wed, 15 May 2024 16:20:02 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, "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

On Thu, May 16, 2024, Kai Huang wrote:
> > > 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?

I highly doubt it, especially given how much the TDP MMU can now do with mmu_lock
held for read.

> 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.

Hmm, a quirk isn't a bad idea.  It suffers the same problems as a memslot flag,
i.e. who knows when it's safe to disable the quirk, but I would hope userspace
would be much, much cautious about disabling a quirk that comes with a massive
disclaimer.

Though I suspect Paolo will shoot this down too ;-)

> 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.

Ya, I'm convinced that adding uAPI is overkill at this point.