Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Mon, Nov 28, 2022 at 11:40:19PM +0100, Borislav Petkov wrote:
On Mon, Nov 28, 2022 at 02:03:58PM -0800, Pawan Gupta wrote:
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3e3230cccaa7..cfc2ed2661fc 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -66,7 +66,7 @@ static DEFINE_MUTEX(spec_ctrl_mutex);
  */
 void write_spec_ctrl_current(u64 val, bool force)
 {
-	if (this_cpu_read(x86_spec_ctrl_current) == val)
+	if (!force && this_cpu_read(x86_spec_ctrl_current) == val)
 		return;
 	this_cpu_write(x86_spec_ctrl_current, val);

Still looks hacky to me.

I think it would be a lot cleaner if MSR_IA32_SPEC_CTRL gets cleaned of
the speculation bits in init_speculation_control() which gets run on
*every* CPU.

So by the time check_bugs() gets to setup stuff, the MSR will be ready
to go regardless.

I.e., something like this (not supposed to work - just to show what I
mean):

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 73cc546e024d..367732c92942 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -993,9 +993,19 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
	 * Intel CPUs, for finer-grained selection of what's available.
	 */
	if (cpu_has(c, X86_FEATURE_SPEC_CTRL)) {
+		u64 msr;
+
		set_cpu_cap(c, X86_FEATURE_IBRS);
		set_cpu_cap(c, X86_FEATURE_IBPB);
		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
+
+		/*
+		 * Clear speculation control settings from a previous kernel
+		 * run, i.e., kexec.
+		 */
+		rdmsrl(MSR_IA32_SPEC_CTRL, msr);
+		if (msr & SPEC_CTRL_MASK)
+			wrmsr (MSR_IA32_SPEC_CTRL, msr & ~SPEC_CTRL_MASK);

Yes thats a cleaner approach, except that the late microcode load will
ruin the MSR:

microcode_reload_late()
  microcode_check()
    get_cpu_cap()
      init_speculation_control()

_______________________________________________
kexec mailing list
kexec@xxxxxxxxxxxxxxxxxxx
http://lists.infradead.org/mailman/listinfo/kexec



[Index of Archives]     [LM Sensors]     [Linux Sound]     [ALSA Users]     [ALSA Devel]     [Linux Audio Users]     [Linux Media]     [Kernel]     [Gimp]     [Yosemite News]     [Linux Media]

  Powered by Linux