Re: [PATCH 0/8] KVM updates for 2.6.20-rc2

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

* Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org> wrote:

> * Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
> 
> > >NOTE: this is not a worry for upstream kernel, it is caused by 
> > >PREEMPT_RT scheduling in previously atomic APIs like kunmap(). But 
> > >KVM used to work pretty nicely in -rt and this problem got introduced 
> > >fairly recently, related to some big-page changes IIRC.
> > 
> > This is not a recent change.  The x86 emulator is fetching an 
> > instruction or a memory operand.
> 
> well, the fact that it's somehow ending up in highmem and thus the 
> kmap becomes nontrivial could be a recent change maybe?

i have worked the problem around via the change below. This affects 
32-bit hosts only, and only on CONFIG_PREEMPT_RT.

with this and the other two patches kvm/vmx works on 32-bit -rt hosts.

	Ingo

Index: linux/drivers/kvm/kvm_main.c
===================================================================
--- linux.orig/drivers/kvm/kvm_main.c
+++ linux/drivers/kvm/kvm_main.c
@@ -628,8 +628,12 @@ raced:
 
 		memset(new.phys_mem, 0, npages * sizeof(struct page *));
 		for (i = 0; i < npages; ++i) {
+#ifdef CONFIG_PREEMPT_RT
+			new.phys_mem[i] = alloc_page(GFP_USER | __GFP_ZERO);
+#else
 			new.phys_mem[i] = alloc_page(GFP_HIGHUSER
 						     | __GFP_ZERO);
+#endif
 			if (!new.phys_mem[i])
 				goto out_free;
 		}

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