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 11:40:27 +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>,
 "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@...il.com" <isaku.yamahata@...il.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Aktas, Erdem" <erdemaktas@...gle.com>,
 "isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.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 11:04 AM, Edgecombe, Rick P wrote:
> On Thu, 2024-03-28 at 09:30 +0800, Xiaoyao Li wrote:
>>> 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.
> 
> MTRR appears to be configured to be type "Fixed" in the TDX module. So the guest could expect to be
> able to use it and be surprised by a #GP.
> 
>          {
>            "MSB": "12",
>            "LSB": "12",
>            "Field Size": "1",
>            "Field Name": "MTRR",
>            "Configuration Details": null,
>            "Bit or Field Virtualization Type": "Fixed",
>            "Virtualization Details": "0x1"
>          },
> 
> If KVM does not support MTRRs in TDX, then it has to return the error somewhere or pretend to
> support it (do nothing but not return an error). Returning an error to the guest would be making up
> arch behavior, and to a lesser degree so would ignoring the WRMSR. 

The root cause is that it's a bad design of TDX to make MTRR fixed1. 
When guest reads MTRR CPUID as 1 while getting #VE on MTRR MSRs, it 
already breaks the architectural behavior. (MAC faces the similar issue 
, MCA is fixed1 as well while accessing MCA related MSRs gets #VE. This 
is why TDX is going to fix them by introducing new feature and make them 
configurable)

> So that is why I lean towards
> returning to userspace and giving the VMM the option to ignore it, return an error to the guest or
> show an error to the user. 

"show an error to the user" doesn't help at all. Because user cannot fix 
it, nor does QEMU.

> If KVM can't support the behavior, better to get an actual error in
> userspace than a mysterious guest hang, right?
What behavior do you mean?

> Outside of what kind of exit it is, do you object to the general plan to punt to userspace?
> 
> Since this is a TDX specific limitation, I guess there is KVM_EXIT_TDX_VMCALL as a general category
> of TDVMCALLs that cannot be handled by KVM.

I just don't see any difference between handling it in KVM and handling 
it in userspace: either a) return error to guest or b) ignore the WRMSR.