Messages in this thread | | | Date | Sun, 6 Sep 2009 07:47:49 +0200 | From | Pavel Machek <> | Subject | Re: [patch 14/48] KVM: Reduce kvm stack usage in kvm_arch_vm_ioctl() |
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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@linux.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@linux.vnet.ibm.com> > Signed-off-by: Avi Kivity <avi@qumranet.com> > Signed-off-by: Greg Kroah-Hartman <gregkh@suse.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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