Messages in this thread | | | Subject | Re: [patch 14/48] KVM: Reduce kvm stack usage in kvm_arch_vm_ioctl() | From | Dave Hansen <> | Date | Sun, 06 Sep 2009 06:43:09 -0700 |
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On Sun, 2009-09-06 at 07:47 +0200, Pavel Machek wrote: > On Fri 2009-09-04 13:07:26, Greg KH wrote: > > 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'...
I guess some context got dropped at some point. I was getting really consistent oopses and goofy memory corruption when running KVM:
http://lkml.org/lkml/2008/3/25/340
Not theoretical at all. I think it cost me a few new gray hairs.
> > 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?
Do we need to give that a better comment? It's explained a bit better here:
http://lkml.org/lkml/2008/7/17/12 http://lkml.org/lkml/2008/7/17/16
Would this comment help?
/* * gcc-3.x will sum the stack usage of two stack variables * if they are declared in two different case blocks. This * union makes it explicit that their stack space can be * shared which greatly reduces stack usage. */
-- Dave
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