linux-kernel - Re: [PATCH v6 10/25] KVM: x86: Add kvm_msr

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Date:   Wed, 15 Nov 2023 17:00:00 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dave.hansen@...el.com>,
        <peterz@...radead.org>, <chao.gao@...el.com>,
        <rick.p.edgecombe@...el.com>, <john.allen@....com>
Subject: Re: [PATCH v6 10/25] KVM: x86: Add kvm_msr_{read,write}() helpers

On 11/3/2023 2:26 AM, Maxim Levitsky wrote:
> On Wed, 2023-11-01 at 12:32 -0700, Sean Christopherson wrote:
>> On Tue, Oct 31, 2023, Maxim Levitsky wrote:
>>> On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
>>>> Wrap __kvm_{get,set}_msr() into two new helpers for KVM usage and use the
>>>> helpers to replace existing usage of the raw functions.
>>>> kvm_msr_{read,write}() are KVM-internal helpers, i.e. used when KVM needs
>>>> to get/set a MSR value for emulating CPU behavior.
>>> I am not sure if I like this patch or not. On one hand the code is cleaner
>>> this way, but on the other hand now it is easier to call kvm_msr_write() on
>>> behalf of the guest.
>>>
>>> For example we also have the 'kvm_set_msr()' which does actually set the msr
>>> on behalf of the guest.
>>>
>>> How about we call the new function kvm_msr_set_host() and rename
>>> kvm_set_msr() to kvm_msr_set_guest(), together with good comments explaning
>>> what they do?
>> LOL, just call me Nostradamus[*] ;-)
>>
>>   : > SSP save/load should go to enter_smm_save_state_64() and rsm_load_state_64(),
>>   : > where other fields of SMRAM are handled.
>>   :
>>   : +1.  The right way to get/set MSRs like this is to use __kvm_get_msr() and pass
>>   : %true for @host_initiated.  Though I would add a prep patch to provide wrappers
>>   : for __kvm_get_msr() and __kvm_set_msr().  Naming will be hard, but I think we
>>                                               ^^^^^^^^^^^^^^^^^^^
>>   : can use kvm_{read,write}_msr() to go along with the KVM-initiated register
>>   : accessors/mutators, e.g. kvm_register_read(), kvm_pdptr_write(), etc.
>>
>> [*] https://lore.kernel.org/all/ZM0YZgFsYWuBFOze@google.com
>>
>>> Also functions like kvm_set_msr_ignored_check(), kvm_set_msr_with_filter() and such,
>>> IMHO have names that are not very user friendly.
>> I don't like the host/guest split because KVM always operates on guest values,
>> e.g. kvm_msr_set_host() in particular could get confusing.
> That makes sense.
>
>> IMO kvm_get_msr() and kvm_set_msr(), and to some extent the helpers you note below,
>> are the real problem.
>>
>> What if we rename kvm_{g,s}et_msr() to kvm_emulate_msr_{read,write}() to make it
>> more obvious that those are the "guest" helpers?  And do that as a prep patch in
>> this series (there aren't _that_ many users).
> Makes sense.

Then I'll modify related code and add the pre-patch in next version, thanks!

>> I'm also in favor of renaming the "inner" helpers, but I think we should tackle
>> those separately.separately
> OK.
>
> Best regards,
> 	Maxim Levitsky
>
>