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

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Date: Fri, 17 May 2024 10:23:23 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>
CC: "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>, "Zhao, Yan Y" <yan.y.zhao@...el.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 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.