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

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Message-ID: <1fe263c0-d6f5-417b-8819-c24dba3dee69@intel.com>
Date: Fri, 17 May 2024 11:16:26 +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: "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 17/05/2024 10:38 am, Edgecombe, Rick P wrote:
> 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?

Dangerous means when "a little wrong to assume the intentions of the 
callers" actually goes wrong.  In other words, a general intention to 
"catch new callers" doesn't make a lot sense to me.

Anyway as said before, it's just my 2 cents, and it's totally up to you.