linux-kernel - Re: [PATCH] docs: clarify KVM related kernel parameters' descriptions

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Date:   Wed, 3 May 2023 15:15:50 +0800
From:   Yan-Jie Wang <yanjiewtw@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andre Przywara <andre.przywara@....com>,
        Avi Kivity <avi@...hat.com>,
        Ching-Chun Huang <jserv@...s.ncku.edu.tw>, trivial@...nel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] docs: clarify KVM related kernel parameters' descriptions

Thank you, Sean.

I will revise the patch accordingly and submit a new version as well
as update the description of kvm-intel.nested to reflect that it is
enabled by default.

On Wed, May 3, 2023 at 1:12 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, May 02, 2023, Yan-Jie Wang wrote:
> > The descriptions of certain KVM related kernel parameters can be
> > ambiguous and confusing. They state 'Disable ...,' which implies that
> > setting them to 1 would disable the associated features or options,
> > when in fact the opposite is true.
> >
> > This commit addresses this issue by revising the descriptions of these
> > parameters to make their intended behavior clear.
>
> Less wrong perhaps, but IMO the actual behavior is still not captured, and from
> a certain perspective the existing "Disable" verbiage better reflects how/when
> most users would actually want to explicitly set a param.
>
> Rather than commit one way or the other, what about reworking the descriptions
> using this as a template?  E.g. state that the param controls something, state
> the default and use that to communicate that 1==enabled, and then, when appropriate,
> clarify that KVM disables (or in some cases ignores) params if hardware doesn't
> support the related feature(s).
>
>         kvm-intel.ept=  [KVM,Intel] Control KVM's use of Extended Page Tables,
>                         a.k.a. Two-Dimensional Page Tables.  Default is 1
>                         (enabled).  Disabled by KVM if hardware lacks support
>                         for EPT.
>
> >
> > Signed-off-by: Yan-Jie Wang <yanjiewtw@...il.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 16 ++++++++--------
> >  1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 9e5bab29685f..cc5abb3d54b9 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2552,10 +2552,10 @@
> >                       on the ratio, such that a page is zapped after 1 hour on average.
> >
> >       kvm-amd.nested= [KVM,AMD] Allow nested virtualization in KVM/SVM.
>
> Eh, I don't see any reason to have this one say "allow" instead of "enable/disable".
>
> > -                     Default is 1 (enabled)
> > +                     Default is 1 (allow)
> >
> > -     kvm-amd.npt=    [KVM,AMD] Disable nested paging (virtualized MMU)
> > -                     for all guests.
> > +     kvm-amd.npt=    [KVM,AMD] Enable nested paging (virtualized MMU)
> > +                     for all guests on capable AMD chips.
> >                       Default is 1 (enabled) if in 64-bit or 32-bit PAE mode.
> >
> >       kvm-arm.mode=
> > @@ -2602,12 +2602,12 @@
> >                       Format: <integer>
> >                       Default: 5
> >
> > -     kvm-intel.ept=  [KVM,Intel] Disable extended page tables
> > +     kvm-intel.ept=  [KVM,Intel] Enable extended page tables
> >                       (virtualized MMU) support on capable Intel chips.
> >                       Default is 1 (enabled)
> >
> >       kvm-intel.emulate_invalid_guest_state=
> > -                     [KVM,Intel] Disable emulation of invalid guest state.
> > +                     [KVM,Intel] Enable emulation of invalid guest state.
> >                       Ignored if kvm-intel.enable_unrestricted_guest=1, as
> >                       guest state is never invalid for unrestricted guests.
> >                       This param doesn't apply to nested guests (L2), as KVM
> > @@ -2615,7 +2615,7 @@
> >                       Default is 1 (enabled)
> >
> >       kvm-intel.flexpriority=
> > -                     [KVM,Intel] Disable FlexPriority feature (TPR shadow).
> > +                     [KVM,Intel] Enable FlexPriority feature (TPR shadow).
> >                       Default is 1 (enabled)
> >
> >       kvm-intel.nested=
> > @@ -2623,7 +2623,7 @@
> >                       Default is 0 (disabled)
>
> Heh, kvm-intel.nested has been enabled by default for quite some time.  Can you
> fix that up too?
>
> >
> >       kvm-intel.unrestricted_guest=
> > -                     [KVM,Intel] Disable unrestricted guest feature
> > +                     [KVM,Intel] Enable unrestricted guest feature
> >                       (virtualized real and unpaged mode) on capable
> >                       Intel chips. Default is 1 (enabled)
> >
> > @@ -2639,7 +2639,7 @@
> >
> >                       Default is cond (do L1 cache flush in specific instances)
> >
> > -     kvm-intel.vpid= [KVM,Intel] Disable Virtual Processor Identification
> > +     kvm-intel.vpid= [KVM,Intel] Enable Virtual Processor Identification
> >                       feature (tagged TLBs) on capable Intel chips.
> >                       Default is 1 (enabled)
> >
> >
> > base-commit: 865fdb08197e657c59e74a35fa32362b12397f58
> > --
> > 2.34.1
> >