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Re: [PATCH v3 2/3] x86/boot: Clear XD_DISABLE from the early boot path
Intel CPUs have a bit in MSR_IA32_MISC_ENABLE that may prevent the NX bit
from being advertised. Clear it unconditionally if we can't find the NX
feature right away on boot.
The conditions for the MSR being read on early boot are (in this order):
* Long Mode is supported
* NX isn't advertised
* The vendor is Intel
The order of checks has been chosen carefully so a virtualized Xen on a
hypervisor that doesn't emulate that MSR (but supports NX) doesn't triple
fault trying to access the non-existing MSR.
With that done, we can remove the XD_DISABLE checks in the intel-specific
init path (as they are already done in early assembly). Keep a printk to
highlight the fact that NX was forcefully enabled.
Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
---
v3:
* In head.S: s/has_nx/got_nx and s/nx_bit/nx
* Style changes in assembly instructions (spaces + width modifiers)
* Big comment in head.S replaced
* Jump directly to .Lno_nx if NX not found and XD_DISABLE not present
* Restored rdmsrl (previously refactored into rdmsr_safe() in v2) and
removed XD_DISABLE clearing in C (as it's now done in head.S).
* Moved printk in intel.c to highlight the XD_DISABLE override even when
done in head.S
---
xen/arch/x86/boot/head.S | 49 ++++++++++++++++++++++----
xen/arch/x86/cpu/intel.c | 16 ++++-----
xen/arch/x86/include/asm/msr-index.h | 2 +-
xen/arch/x86/include/asm/x86-vendors.h | 6 ++--
4 files changed, 53 insertions(+), 20 deletions(-)
@mantainers
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