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x86/speculation/l1tf: Drop the swap storage limit restriction when l1tf=off 

Swap storage is restricted to max_swapfile_size (~16TB on x86_64) whenever
the system is deemed affected by L1TF vulnerability. Even though the limit
is quite high for most deployments it seems to be too restrictive for
deployments which are willing to live with the mitigation disabled.

We have a customer to deploy 8x 6,4TB PCIe/NVMe SSD swap devices which is
clearly out of the limit.

Drop the swap restriction when l1tf=off is specified. It also doesn't make
much sense to warn about too much memory for the l1tf mitigation when it is
forcefully disabled by the administrator.

[ tglx: Folded the documentation delta change ]

Fixes: 377eeaa8e1 ("x86/speculation/l1tf: Limit swap file size to MAX_PA/2")
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: <linux-mm@kvack.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20181113184910.26697-1-mhocko@kernel.org
This commit is contained in:
Michal Hocko 2018-11-13 19:49:10 +01:00 committed by Thomas Gleixner
parent aa77bfb354
commit 5b5e4d623e
4 changed files with 11 additions and 3 deletions
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3
Documentation/admin-guide/kernel-parameters.txt
View File

@ -2095,6 +2095,9 @@
off
Disables hypervisor mitigations and doesn't
emit any warnings.
It also drops the swap size and available
RAM limit restriction on both hypervisor and
bare metal.
Default is 'flush'.

6
Documentation/admin-guide/l1tf.rst
View File

@ -405,6 +405,9 @@ time with the option "l1tf=". The valid arguments for this option are:
off Disables hypervisor mitigations and doesn't emit any
warnings.
It also drops the swap size and available RAM limit restrictions
on both hypervisor and bare metal.
============ =============================================================
The default is 'flush'. For details about L1D flushing see :ref:`l1d_flush`.
@ -576,7 +579,8 @@ Default mitigations
The kernel default mitigations for vulnerable processors are:
- PTE inversion to protect against malicious user space. This is done
unconditionally and cannot be controlled.
unconditionally and cannot be controlled. The swap storage is limited
to ~16TB.
- L1D conditional flushing on VMENTER when EPT is enabled for
a guest.

3
arch/x86/kernel/cpu/bugs.c
View File

@ -1002,7 +1002,8 @@ static void __init l1tf_select_mitigation(void)
#endif
half_pa = (u64)l1tf_pfn_limit() << PAGE_SHIFT;
if (e820__mapped_any(half_pa, ULLONG_MAX - half_pa, E820_TYPE_RAM)) {
if (l1tf_mitigation != L1TF_MITIGATION_OFF &&
e820__mapped_any(half_pa, ULLONG_MAX - half_pa, E820_TYPE_RAM)) {
pr_warn("System has more than MAX_PA/2 memory. L1TF mitigation not effective.\n");
pr_info("You may make it effective by booting the kernel with mem=%llu parameter.\n",
half_pa);

2
arch/x86/mm/init.c
View File

@ -931,7 +931,7 @@ unsigned long max_swapfile_size(void)
pages = generic_max_swapfile_size();
if (boot_cpu_has_bug(X86_BUG_L1TF)) {
if (boot_cpu_has_bug(X86_BUG_L1TF) && l1tf_mitigation != L1TF_MITIGATION_OFF) {
/* Limit the swap file size to MAX_PA/2 for L1TF workaround */
unsigned long long l1tf_limit = l1tf_pfn_limit();
/*