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[PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround in a hypervisor

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  [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround in  a hypervisor Mikulas Patocka <mpatocka@redhat.com> - 2017-07-12 09:30 +0200
    Re: [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround  in a hypervisor Borislav Petkov <bp@alien8.de> - 2017-07-12 15:50 +0200
      Re: [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround  in a hypervisor Mikulas Patocka <mpatocka@redhat.com> - 2017-07-13 04:30 +0200
        Re: [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround  in a hypervisor Borislav Petkov <bp@alien8.de> - 2017-07-13 08:40 +0200

#1685566 — [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround in a hypervisor

When I start a virtual machine, kvm complains about accesses to an unknown
MSR ("vcpu0 unhandled rdmsr: 0xc001100d"). This is caused by the code in
init_amd_k8 that tries to work around a BIOS bug that incorrectly sets
lahf_lm.

This patch disables this workaround when running in a hypervisor.

Note that the processors for which this erratum applies do not support
virtualization at all, so if we are running in a hypervisor, we can be
sure that the workaronud doesn't apply.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 arch/x86/kernel/cpu/amd.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Index: linux-2.6/arch/x86/kernel/cpu/amd.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/cpu/amd.c
+++ linux-2.6/arch/x86/kernel/cpu/amd.c
@@ -627,8 +627,12 @@ static void init_amd_k8(struct cpuinfo_x
 	 * Some BIOSes incorrectly force this feature, but only K8 revision D
 	 * (model = 0x14) and later actually support it.
 	 * (AMD Erratum #110, docId: 25759).
+	 *
+	 * Don't apply this workaround in a virtual machine because kvm
+	 * complains about reads from an unknown MSR.
 	 */
-	if (c->x86_model < 0x14 && cpu_has(c, X86_FEATURE_LAHF_LM)) {
+	if (c->x86_model < 0x14 && cpu_has(c, X86_FEATURE_LAHF_LM) &&
+	    !boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
 		clear_cpu_cap(c, X86_FEATURE_LAHF_LM);
 		if (!rdmsrl_amd_safe(0xc001100d, &value)) {
 			value &= ~BIT_64(32);

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#1685782 — Re: [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround in a hypervisor

On Wed, Jul 12, 2017 at 03:19:56AM -0400, Mikulas Patocka wrote:
> When I start a virtual machine, kvm complains about accesses to an unknown
> MSR ("vcpu0 unhandled rdmsr: 0xc001100d"). This is caused by the code in
> init_amd_k8 that tries to work around a BIOS bug that incorrectly sets
> lahf_lm.

No need - there are a lot of qemu configurations which cause the
unhandled MSR warning so no need to pollute the kernel unnecessarily.
The warning is harmless at that.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--

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#1686189 — Re: [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround in a hypervisor

On Wed, 12 Jul 2017, Borislav Petkov wrote:

> On Wed, Jul 12, 2017 at 03:19:56AM -0400, Mikulas Patocka wrote:
> > When I start a virtual machine, kvm complains about accesses to an unknown
> > MSR ("vcpu0 unhandled rdmsr: 0xc001100d"). This is caused by the code in
> > init_amd_k8 that tries to work around a BIOS bug that incorrectly sets
> > lahf_lm.
> 
> No need - there are a lot of qemu configurations which cause the
> unhandled MSR warning so no need to pollute the kernel unnecessarily.
> The warning is harmless at that.

It is not strictly needed, but it is annoying. I work in the console and 
it is annoying to see these "unhandled rdmsr" messages printed over the 
screen every time I reboot a virtual machine.

Or do you thnik that the code that prints this warning should be removed 
from the kernel instead?

Mikulas

> -- 
> Regards/Gruss,
>     Boris.
> 
> ECO tip #101: Trim your mails when you reply.
> --

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#1686283 — Re: [PATCH] x86/kernel/cpu/amd.c: don't apply the lahf_lm workaround in a hypervisor

On Wed, Jul 12, 2017 at 10:27:44PM -0400, Mikulas Patocka wrote:
> It is not strictly needed, but it is annoying. I work in the console and 
> it is annoying to see these "unhandled rdmsr" messages printed over the 
> screen every time I reboot a virtual machine.

Those messages are already pr_debug_ratelimited(). Fix your loglevel to
not see them.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--

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