linux-kernel - Re: [PATCH 1/5] kvm: add exit_to_guest_mode() and enter_from_guest

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Date:   Thu, 13 Jan 2022 11:55:11 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, aleksandar.qemu.devel@...il.com,
        alexandru.elisei@....com, anup.patel@....com,
        aou@...s.berkeley.edu, atish.patra@....com,
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        catalin.marinas@....com, chenhuacai@...nel.org,
        dave.hansen@...ux.intel.com, david@...hat.com,
        frankja@...ux.ibm.com, frederic@...nel.org, gor@...ux.ibm.com,
        hca@...ux.ibm.com, imbrenda@...ux.ibm.com, james.morse@....com,
        jmattson@...gle.com, joro@...tes.org, kvm@...r.kernel.org,
        mingo@...hat.com, mpe@...erman.id.au, nsaenzju@...hat.com,
        palmer@...belt.com, paulmck@...nel.org, paulus@...ba.org,
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        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH 1/5] kvm: add exit_to_guest_mode() and enter_from_guest_mode()

On Thu, 13 Jan 2022 11:01:30 +0000,
Mark Rutland <mark.rutland@....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@....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 again,

	M.

-- 
Without deviation from the norm, progress is not possible.