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Message-ID: <20090906054749.GB1431@ucw.cz>
Date: Sun, 6 Sep 2009 07:47:49 +0200
From: Pavel Machek <pavel@....cz>
To: Greg KH <gregkh@...e.de>
Cc: linux-kernel@...r.kernel.org, stable@...nel.org,
stable-review@...nel.org, torvalds@...ux-foundation.org,
akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
Avi Kivity <avi@...hat.com>,
Dave Hansen <dave@...ux.vnet.ibm.com>,
Avi Kivity <avi@...ranet.com>
Subject: Re: [patch 14/48] KVM: Reduce kvm stack usage in
kvm_arch_vm_ioctl()
On Fri 2009-09-04 13:07:26, Greg KH wrote:
>
> 2.6.27-stable review patch. If anyone has any objections, please let us know.
>
> ------------------
> From: Dave Hansen <dave@...ux.vnet.ibm.com>
>
> (cherry picked from commit f0d662759a2465babdba1160749c446648c9d159)
>
> On my machine with gcc 3.4, kvm uses ~2k of stack in a few
> select functions. This is mostly because gcc fails to
> notice that the different case: statements could have their
> stack usage combined. It overflows very nicely if interrupts
> happen during one of these large uses.
>
> This patch uses two methods for reducing stack usage.
> 1. dynamically allocate large objects instead of putting
> on the stack.
> 2. Use a union{} member for all of the case variables. This
> tricks gcc into combining them all into a single stack
> allocation. (There's also a comment on this)
Are the 'reduce stack usage' patches suitable for stable? The rules
said that fix must be for 'serious problem', not 'theoretical
issue'...
>
> Signed-off-by: Dave Hansen <dave@...ux.vnet.ibm.com>
> Signed-off-by: Avi Kivity <avi@...ranet.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
> ---
> arch/x86/kvm/x86.c | 72 +++++++++++++++++++++++++++++++++--------------------
> 1 file changed, 45 insertions(+), 27 deletions(-)
>
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1630,6 +1630,15 @@ long kvm_arch_vm_ioctl(struct file *filp
> struct kvm *kvm = filp->private_data;
> void __user *argp = (void __user *)arg;
> int r = -EINVAL;
> + /*
> + * This union makes it completely explicit to gcc-3.x
> + * that these two variables' stack usage should be
> + * combined, not added together.
> + */
> + union {
> + struct kvm_pit_state ps;
> + struct kvm_memory_alias alias;
> + } u;
>
> switch (ioctl) {
> case KVM_SET_TSS_ADDR:
...plus this is really ugly hack. Just declare the variable inside the
case block that needs it?
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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