On 05/07/2014 07:56 AM, Paul Mackerras wrote:
On Sun, May 04, 2014 at 10:56:08PM +0530, Aneesh Kumar K.V wrote:
With debug option "sleep inside atomic section checking" enabled we get
the below WARN_ON during a PR KVM boot. This is because upstream now
have PREEMPT_COUNT enabled even if we have preempt disabled. Fix the
warning by adding preempt_disable/enable around floating point and altivec
enable.
This worries me a bit. In this code:
if (msr & MSR_FP) {
+ preempt_disable();
enable_kernel_fp();
load_fp_state(&vcpu->arch.fp);
t->fp_save_area = &vcpu->arch.fp;
+ preempt_enable();
What would happen if we actually did get preempted at this point?
Wouldn't we lose the FP state we just loaded?
In other words, how come we're not already preempt-disabled at this
point?
This is probably because we're trying to confuse Linux :). The entry
path happens with interrupts hard disabled, but preempt enabled so that
Linux doesn't consider the guest time as non-preemptible. That's the
only call I could find where preempt is logically enabled (though it
really isn't).
Alex
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