linux-kernel - Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn

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Message-ID: <00d2cfe84f67dde1d9cbdcb2a2d907354e2d704a.camel@intel.com>
Date: Thu, 16 May 2024 22:38:19 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>
CC: "Zhao, Yan Y" <yan.y.zhao@...el.com>, "sagis@...gle.com"
	<sagis@...gle.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "Aktas, Erdem"
	<erdemaktas@...gle.com>
Subject: Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn_range() is
 called for TDX

On Fri, 2024-05-17 at 10:23 +1200, Huang, Kai wrote:
> On 17/05/2024 9:46 am, Edgecombe, Rick P wrote:
> > On Tue, 2024-05-14 at 17:59 -0700, Rick Edgecombe wrote:
> > > 
> > > For lack of a better method currently, use kvm_gfn_shared_mask() to
> > > determine if private memory cannot be zapped (as in TDX, the only VM type
> > > that sets it).
> > 
> > Trying to replace kvm_gfn_shared_mask() with something appropriate, I saw
> > that
> > SNP actually uses this function:
> > https://lore.kernel.org/kvm/20240501085210.2213060-12-michael.roth@amd.com/
> > 
> > So trying to have a helper that says "The VM can't zap and refault in memory
> > at
> > will" won't cut it. I guess there would have to be some more specific. I'm
> > thinking to just drop this patch instead.
> 
> Or KVM_BUG_ON() in the callers by explicitly checking VM type being TDX 
> as I mentioned before.
> 
> Having such checking in a generic function like this is just dangerous 
> and not flexible.
> 
> Just my 2 cents, though.

As I said before, the point is to catch new callers. I see how it's a little
wrong to assume the intentions of the callers, but I don't see how it's
dangerous. Can you explain?

But you just reminded me that, yes, we can probably just check the vm_type here:
https://lore.kernel.org/kvm/f64c7da52a849cd9697b944769c200dfa3ee7db7.camel@intel.com/