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SubjectRe: [PATCH 1/5] kvm: add exit_to_guest_mode() and enter_from_guest_mode()
On Thu, Jan 13, 2022 at 11:55:11AM +0000, Marc Zyngier wrote:
> On Thu, 13 Jan 2022 11:01:30 +0000,
> Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > On Tue, Jan 11, 2022 at 05:54:59PM +0000, Marc Zyngier wrote:
> > > Hi Mark,
> > >
> > > On Tue, 11 Jan 2022 15:35:35 +0000,
> > > Mark Rutland <mark.rutland@arm.com> wrote:
>
> [...]
>
> > > > +/*
> > > > + * Enter guest context and enter an RCU extended quiescent state.
> > > > + *
> > > > + * This should be the last thing called before entering the guest, and must be
> > > > + * called after any potential use of RCU (including any potentially
> > > > + * instrumented code).
> > >
> > > nit: "the last thing called" is terribly ambiguous. Any architecture
> > > obviously calls a ****load of stuff after this point. Should this be
> > > 'the last thing involving RCU' instead?
> >
> > I agree this is unclear and I struggled to fing good wording for this. Is the
> > following any better?
> >
> > /*
> > * Enter guest context and enter an RCU extended quiescent state.
> > *
> > * Between guest_context_enter_irqoff() and guest_context_exit_irqoff() it is
> > * unsafe to use any code which may directly or indirectly use RCU, tracing
> > * (including IRQ flag tracing), or lockdep. All code in this period must be
> > * non-instrumentable.
> > */
> >
> > If that's good I can add similar to guest_context_exit_irqoff().
>
> Yes, that's much clearer, thanks.
>
> >
> > [...]
> >
> > > > +/**
> > > > + * exit_to_guest_mode - Fixup state when exiting to guest mode
> > > > + *
> > > > + * This is analagous to exit_to_user_mode(), and ensures we perform the
> > > > + * following in order:
> > > > + *
> > > > + * 1) Trace interrupts on state
> > > > + * 2) Invoke context tracking if enabled to adjust RCU state
> > > > + * 3) Tell lockdep that interrupts are enabled
> > >
> > > nit: or rather, are about to be enabled? Certainly on arm64, the
> > > enable happens much later, right at the point where we enter the guest
> > > for real.
> >
> > True; I'd cribbed the wording from the comment block above exit_to_user_mode(),
> > but I stripped the context that made that clear. I'll make that:
> >
> > /**
> > * exit_to_guest_mode - Fixup state when exiting to guest mode
> > *
> > * Entry to a guest will enable interrupts, but the kernel state is
> > * interrupts disabled when this is invoked. Also tell RCU about it.
> > *
> > * 1) Trace interrupts on state
> > * 2) Invoke context tracking if enabled to adjust RCU state
> > * 3) Tell lockdep that interrupts are enabled
> > *
> > * Invoked from architecture specific code before entering a guest.
> > * Must be called with interrupts disabled and the caller must be
> > * non-instrumentable.
> > * The caller has to invoke guest_timing_enter_irqoff() before this.
> > *
> > * Note: this is analagous to exit_to_user_mode().
>
> nit: analogous
>
> > */
> >
> > ... with likewise for enter_from_guest_mode(), if that's clear enough?
>
> Yes, that's great.

Thanks; I've pushed out an updated branch with those changes (including the
typo fixes). I'll wait until next week before sending out a v2 since I don't
think that meaningfully affects the arch bits for other architectures.

Mark.

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