linux-kernel - Re: [PATCH RFC 2/2] paravirt: clean up lazy mode handling

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Date:	Mon, 15 Oct 2007 22:54:32 -0300
From:	"Glauber de Oliveira Costa" <glommer@...il.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Andi Kleen" <ak@...e.de>, "Zachary Amsden" <zach@...are.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	"Avi Kivity" <avi@...ranet.com>,
	"Anthony Liguori" <anthony@...emonkey.ws>,
	"Nakajima, Jun" <jun.nakajima@...el.com>
Subject: Re: [PATCH RFC 2/2] paravirt: clean up lazy mode handling

On 10/12/07, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> [ Changes since last post: fixed up lguest ]
>
> Currently, the set_lazy_mode pv_op is overloaded with 5 functions:
>  1. enter lazy cpu mode
>  2. leave lazy cpu mode
>  3. enter lazy mmu mode
>  4. leave lazy mmu mode
>  5. flush pending batched operations
>
> This complicates each paravirt backend, since it needs to deal with
> all the possible state transitions, handling flushing, etc. In
> particular, flushing is quite distinct from the other 4 functions, and
> seems to just cause complication.
>
> This patch removes the set_lazy_mode operation, and adds "enter" and
> "leave" lazy mode operations on mmu_ops and cpu_ops.  All the logic
> associated with enter and leaving lazy states is now in common code
> (basically BUG_ONs to make sure that no mode is current when entering
> a lazy mode, and make sure that the mode is current when leaving).
> Also, flush is handled in a common way, by simply leaving and
> re-entering the lazy mode.
>
> The result is that the Xen, lguest and VMI lazy mode implementations
> are much simpler.
>
> Signed-off-by: Jeremy Fitzhardinge <jeremy@...source.com>
> Cc: Andi Kleen <ak@...e.de>
> Cc: Zach Amsden <zach@...are.com>
> Cc: Rusty Russell <rusty@...tcorp.com.au>
> Cc: Avi Kivity <avi@...ranet.com>
> Cc: Anthony Liguory <aliguori@...ibm.com>
> Cc: "Glauber de Oliveira Costa" <glommer@...il.com>
> Cc: "Nakajima, Jun" <jun.nakajima@...el.com>
>
> ---
>  arch/i386/kernel/paravirt.c |   58 +++++++++++++++++++++++++++++++++++++++----
>  arch/i386/kernel/vmi.c      |   45 +++++++++++++++++++--------------
>  arch/i386/xen/enlighten.c   |   44 ++++++++++----------------------
>  arch/i386/xen/mmu.c         |    2 -
>  arch/i386/xen/multicalls.h  |    2 -
>  arch/i386/xen/xen-ops.h     |    7 -----
>  drivers/lguest/lguest.c     |   34 ++++++++-----------------
>  include/asm-i386/paravirt.h |   52 ++++++++++++++++++++++++--------------
>  8 files changed, 140 insertions(+), 104 deletions(-)
>
> ===================================================================
> --- a/arch/i386/kernel/paravirt.c
> +++ b/arch/i386/kernel/paravirt.c
> @@ -266,6 +266,49 @@ int paravirt_disable_iospace(void)
>         }
>
>         return ret;
> +}
> +
> +static DEFINE_PER_CPU(enum paravirt_lazy_mode, paravirt_lazy_mode) = PARAVIRT_LAZY_NONE;
> +
> +static inline void enter_lazy(enum paravirt_lazy_mode mode)
> +{
> +       BUG_ON(x86_read_percpu(paravirt_lazy_mode) != PARAVIRT_LAZY_NONE);
> +       BUG_ON(preemptible());
Wouldn't it be better to WARN_ON, and simply not entering lazy mode?
It does not sound like a fatal condition.

> +void paravirt_leave_lazy(enum paravirt_lazy_mode mode)
> +{
> +       BUG_ON(x86_read_percpu(paravirt_lazy_mode) != mode);
> +       BUG_ON(preemptible());

Although this one seems like a fatal condition ;-)

> +void paravirt_enter_lazy_mmu(void)
> +{
> +       enter_lazy(PARAVIRT_LAZY_MMU);
> +}
> +
> +void paravirt_leave_lazy_mmu(void)
> +{
> +       paravirt_leave_lazy(PARAVIRT_LAZY_MMU);
> +}
> +
> +void paravirt_enter_lazy_cpu(void)
> +{
> +       enter_lazy(PARAVIRT_LAZY_CPU);
> +}
> +
> +void paravirt_leave_lazy_cpu(void)
> +{
> +       paravirt_leave_lazy(PARAVIRT_LAZY_CPU);
> +}
> +
> +enum paravirt_lazy_mode paravirt_get_lazy_mode(void)
> +{
> +       return x86_read_percpu(paravirt_lazy_mode);
>  }

I am concerned that this is 32-bit specific.
But hey: We could wrap it here, but the best solution may be just to
define this macro for 64-bit, and make it everyone benefits. So yeah,
this is a concern here, but I don't think anything should be changed
in this patch to address it so... so... ok ;-)

-- 
Glauber de Oliveira Costa.
"Free as in Freedom"
http://glommer.net

"The less confident you are, the more serious you have to act."
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