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    SubjectRe: [patch 14/48] KVM: Reduce kvm stack usage in kvm_arch_vm_ioctl()
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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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