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

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Message-ID: <1e9921f687abe09d8797e6fb83760acf970f344e.camel@redhat.com>
Date:   Thu, 02 Nov 2023 20:26:05 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 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.

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

OK.

> 

Best regards,
	Maxim Levitsky