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

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Date:   Wed, 1 Nov 2023 12:32:25 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, 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 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.

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).

I'm also in favor of renaming the "inner" helpers, but I think we should tackle
those separately.separately