linux-kernel - Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

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Date: Thu, 28 Mar 2024 09:30:11 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "Gao, Chao" <chao.gao@...el.com>, "Yamahata, Isaku"
 <isaku.yamahata@...el.com>
Cc: "Zhang, Tina" <tina.zhang@...el.com>,
 "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>,
 "Chen, Bo2" <chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
 "isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Aktas, Erdem" <erdemaktas@...gle.com>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>, "Yuan, Hang"
 <hang.yuan@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for
 unsupported cases

On 3/28/2024 9:06 AM, Edgecombe, Rick P wrote:
> On Thu, 2024-03-28 at 08:58 +0800, Xiaoyao Li wrote:
>>> How so? Userspace needs to learn to create a TD first.
>>
>> The current ABI of KVM_EXIT_X86_RDMSR/WRMSR is that userspace itself
>> sets up MSR fitler at first, then it will get such EXIT_REASON when
>> guest accesses the MSRs being filtered.
>>
>> If you want to use this EXIT reason, then you need to enforce userspace
>> setting up the MSR filter. How to enforce?
> 
> I think Isaku's proposal was to let userspace configure it.
> 
> For the sake of conversation, what if we don't enforce it? The downside of not enforcing it is that
> we then need to worry about code paths in KVM the MTRRs would call. But what goes wrong
> functionally? If userspace doesn't fully setup a TD things can go wrong for the TD.
> 
> A plus side of using the MSR filter stuff is it reuses existing functionality.
> 
>>   If not enforce, but exit with
>> KVM_EXIT_X86_RDMSR/WRMSR no matter usersapce sets up MSR filter or not.
>> Then you are trying to introduce divergent behavior in KVM.
> 
> The current ABI of KVM_EXIT_X86_RDMSR when TDs are created is nothing. So I don't see how this is
> any kind of ABI break. If you agree we shouldn't try to support MTRRs, do you have a different exit
> reason or behavior in mind?

Just return error on TDVMCALL of RDMSR/WRMSR on TD's access of MTRR MSRs.