On 09/29/2015 06:20 PM, Eric W. Biederman wrote: > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > >> On Tue, Sep 29, 2015 at 1:35 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>> >>> Does anyone know what happens if you stick a non-accessed segment in >>> the GDT, map the GDT RO, and access it? >> >> You should get a #PF, as you guess, but go ahead and test it if you >> want to make sure. > > I tested this by accident once when workinng on what has become known > as coreboot. Early in boot with your GDT in a EEPROM switching from > real mode to 32bit protected mode causes a write and locks up the > machine when the hardware declines the write to the GDT to set the > accessed bit. As I recall the write kept being retried and retried and > retried... > > Setting the access bit in the GDT cleared up the problem and I did not > look back. > > Way up in 64bit mode something might be different, but I don't know why > cpu designeres would waste the silicon. > This is totally different from a TLB violation. In your case, the write goes through as far as the CPU is concerned, but when the data is fetched back, it hasn't changed. A write to a TLB-protected location will #PF. -hpa -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
