pci-gart_64.c: could we get better explanation?

[Pavel Machek]

Hi!

+++ b/arch/x86/kernel/pci-gart_64.c
@@ -774,6 +778,7 @@ #endif
 
        /*
         * Try to workaround a bug (thanks to BenH)
         * Set unmapped entries to a scratch page instead of 0.
         * Any prefetches that hit unmapped entries won't get an bus abort

Could someone explain? Which bug is being worked around? CPUs
prefetching over NULL?
								Pavel
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