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

[Jürgen Groß]

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.


Juergen

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