Re: [PATCH v2 0/6] KVM/x86: Drop "1" as MSR emulation return value

[Juergen Gross <jgross@suse.com>]

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On 28.05.26 17:50, Jürgen Groß wrote:
> On 28.05.26 15:21, Sean Christopherson wrote:
>> On Thu, May 28, 2026, Jürgen Groß wrote:
>>> On 28.05.26 15:09, Sean Christopherson wrote:
>>>> On Thu, May 28, 2026, Juergen Gross wrote:
>>>>> Please disregard this series, there is one complication sashiko made me
>>>>> aware of.
>>>>
>>>> Sashiko beat me to the punch. :-)
>>>>
>>>> See commit 2368048bf5c2 ("KVM: x86: Signal #GP, not -EPERM, on bad 
>>>> WRMSR(MCi_CTL/STATUS)")
>>>> for a real world example of how things can and will go wrong.
>>>
>>> Yeah, with Sashiko's pointer it was easy to spot.
>>>
>>> Question now is whether the already existing cases of -errno passed as return
>>> value are wrong or on purpose.
>>
>> What are the existing cases?
>>
>>> If the latter, there should be a comment for
>>> that, otherwise they need to be fixed..
>>>
>>> Disentangling the MSR emulation return values from the "normal" ones ("return
>>> to guest"/"return to user mode") will be quite interesting with the overloaded
>>> semantics of "1".
>>
>> LOL, "interesting".
> 
> What do you think about the following idea:
> 
> Lets pass struct msr_info * down to all functions which get their return
> value passed up. Then extend msr_info with a bool "return_to_guest" (valid
> only if !host_initiated), which should be set instead of passing "1" up to
> the caller (probably using an inline helper). Then the return value could
> be 0 or -errno, and after MSR emulation the return_to_guest indicator can
> be tested if needed.

In the end I think it is less error prone to define a struct kvm_msr_ret_t
used as return type, consisting of an int and a bool with the same semantics
as above. Helpers will still be a good idea IMHO.

I'll have a try how it looks like.


Juergen

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