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Date: Mon, 29 Apr 2024 14:45:25 +1200
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH 0/3] KVM: x86: Fix supported VM_TYPES caps
To: Sean Christopherson <seanjc@google.com>
CC: Xiaoyao Li <xiaoyao.li@intel.com>, "kvm@vger.kernel.org"
	<kvm@vger.kernel.org>, "pbonzini@redhat.com" <pbonzini@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20240423165328.2853870-1-seanjc@google.com>
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From: "Huang, Kai" <kai.huang@intel.com>
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On 27/04/2024 3:47 am, Sean Christopherson wrote:
> On Fri, Apr 26, 2024, Kai Huang wrote:
>> On Thu, 2024-04-25 at 07:30 -0700, Sean Christopherson wrote:
>>> On Thu, Apr 25, 2024, Xiaoyao Li wrote:
>>>> On 4/24/2024 12:53 AM, Sean Christopherson wrote:
>>>>> Fix a goof where KVM fails to re-initialize the set of supported VM types,
>>>>> resulting in KVM overreporting the set of supported types when a vendor
>>>>> module is reloaded with incompatible settings.  E.g. unload kvm-intel.ko,
>>>>> reload with ept=0, and KVM will incorrectly treat SW_PROTECTED_VM as
>>>>> supported.
>>>>
>>>> Hah, this reminds me of the bug of msrs_to_save[] and etc.
>>>>
>>>>     7a5ee6edb42e ("KVM: X86: Fix initialization of MSR lists")
>>>
>>> Yeah, and we had the same bug with allow_smaller_maxphyaddr
>>>
>>>    88213da23514 ("kvm: x86: disable the narrow guest module parameter on unload")
>>>
>>> If the side effects of linking kvm.ko into kvm-{amd,intel}.ko weren't so painful
>>> for userspace,
>>>
>>
>> Do we have any real side effects for _userspace_ here?
> 
> kvm.ko ceasing to exist, and "everything" being tied to the vendor module is the
> big problem.  E.g. params from the kernel command line for kvm.??? will become
> ineffective, etc.  Some of that can be handled in the kernel, e.g. KVM can create
> a sysfs symlink so that the accesses through sysfs continue to work, but AFAIK
> params don't supporting such aliasing/links.
> 
> I don't think there are any deal breakers, but I don't expect it to Just Work either.

Perhaps we can make the kvm.ko as a dummy module which only keeps the 
module parameters for backward compatibility?