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Date: Thu, 28 Mar 2024 09:30:11 +0800
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Subject: Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for
 unsupported cases
To: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
 "Gao, Chao" <chao.gao@intel.com>, "Yamahata, Isaku"
 <isaku.yamahata@intel.com>
Cc: "Zhang, Tina" <tina.zhang@intel.com>,
 "isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>,
 "seanjc@google.com" <seanjc@google.com>, "Huang, Kai" <kai.huang@intel.com>,
 "Chen, Bo2" <chen.bo@intel.com>, "sagis@google.com" <sagis@google.com>,
 "isaku.yamahata@linux.intel.com" <isaku.yamahata@linux.intel.com>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "Aktas, Erdem" <erdemaktas@google.com>,
 "pbonzini@redhat.com" <pbonzini@redhat.com>, "Yuan, Hang"
 <hang.yuan@intel.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>
References: <96fcb59cd53ece2c0d269f39c424d087876b3c73.camel@intel.com>
 <20240325190525.GG2357401@ls.amr.corp.intel.com>
 <5917c0ee26cf2bb82a4ff14d35e46c219b40a13f.camel@intel.com>
 <20240325221836.GO2357401@ls.amr.corp.intel.com>
 <20240325231058.GP2357401@ls.amr.corp.intel.com>
 <edcfc04cf358e6f885f65d881ef2f2165e059d7e.camel@intel.com>
 <20240325233528.GQ2357401@ls.amr.corp.intel.com>
 <ZgIzvHKobT2K8LZb@chao-email>
 <20db87741e356e22a72fadeda8ab982260f26705.camel@intel.com>
 <ZgKt6ljcmnfSbqG/@chao-email>
 <20240326174859.GB2444378@ls.amr.corp.intel.com>
 <481141ba-4bdf-40f3-9c32-585281c7aa6f@intel.com>
 <34ca8222fcfebf1d9b2ceb20e44582176d2cef24.camel@intel.com>
 <873263e8-371a-47a0-bba3-ed28fcc1fac0@intel.com>
 <e0ac83c57da3c853ffc752636a4a50fe7b490884.camel@intel.com>
 <5f07dd6c-b06a-49ed-ab16-24797c9f1bf7@intel.com>
 <d7a0ed833909551c24bf1c2c52b8955d75359249.camel@intel.com>
Content-Language: en-US
From: Xiaoyao Li <xiaoyao.li@intel.com>
In-Reply-To: <d7a0ed833909551c24bf1c2c52b8955d75359249.camel@intel.com>
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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.