[PATCH -v2] x86/CPU/AMD: Improve the erratum 1386 workaround

Borislav Petkov
2024-03-24T20:05:42Z

On Thu, Mar 21, 2024 at 04:21:39PM +0100, Maciej S. Szmigiero wrote:
> On 21.03.2024 13:05, Borislav Petkov wrote:
> > From: "Borislav Petkov (AMD)" <bp@alien8.de>
> >
> > Disable XSAVES only on machines which haven't loaded the microcode
> > revision containing the erratum fix.
> >
> > This will come in handy when running archaic OSes as guests. OSes whose
> > brilliant programmers thought that CPUID is overrated and one should not
> > query it but use features directly, ala shoot first, ask questions
> > later... but only if you're alive after the shooting.
> >
> > Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
> > Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > Cc: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> > ---
>
> Tested-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

Yeah, thanks but still not good enough. Turns out we already have the
min microcode revision testing functionality so there's no need for me
to reinvent the wheel.

No harm no foul, tho, the stuff in those two previous patches I will use
in the microcode loader soon for simplifying the loading.

Here's one *final* variant, I promise! :-)

Sorry about that.

---
From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Sun, 24 Mar 2024 20:51:35 +0100
Subject: [PATCH -v2] x86/CPU/AMD: Improve the erratum 1386 workaround

Disable XSAVES only on machines which haven't loaded the microcode
revision containing the erratum fix.

This will come in handy when running archaic OSes as guests. OSes whose
brilliant programmers thought that CPUID is overrated and one should not
query it but use features directly, ala shoot first, ask questions
later... but only if you're alive after the shooting.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
---
arch/x86/include/asm/cpu_device_id.h | 8 ++++++++
arch/x86/kernel/cpu/amd.c | 12 ++++++++++++
2 files changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/cpu_device_id.h b/arch/x86/include/asm/cpu_device_id.h
index eb8fcede9e3b..bf4e065cf1e2 100644
--- a/arch/x86/include/asm/cpu_device_id.h
+++ b/arch/x86/include/asm/cpu_device_id.h
@@ -190,6 +190,14 @@ struct x86_cpu_desc {
.x86_microcode_rev = (revision), \
}

+#define AMD_CPU_DESC(fam, model, stepping, revision) { \
+ .x86_family = (fam), \
+ .x86_vendor = X86_VENDOR_AMD, \
+ .x86_model = (model), \
+ .x86_stepping = (stepping), \
+ .x86_microcode_rev = (revision), \
+}
+
extern const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id *match);
extern bool x86_cpu_has_min_microcode_rev(const struct x86_cpu_desc *table);

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 6d8677e80ddb..873f0fdc2ef8 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -13,6 +13,7 @@
#include <asm/apic.h>
#include <asm/cacheinfo.h>
#include <asm/cpu.h>
+#include <asm/cpu_device_id.h>
#include <asm/spec-ctrl.h>
#include <asm/smp.h>
#include <asm/numa.h>
@@ -802,6 +803,11 @@ static void init_amd_bd(struct cpuinfo_x86 *c)
clear_rdrand_cpuid_bit(c);
}

+static const struct x86_cpu_desc erratum_1386_microcode[] = {
+ AMD_CPU_DESC(0x17, 0x1, 0x2, 0x0800126e),
+ AMD_CPU_DESC(0x17, 0x31, 0x0, 0x08301052),
+};
+
static void fix_erratum_1386(struct cpuinfo_x86 *c)
{
/*
@@ -811,7 +817,13 @@ static void fix_erratum_1386(struct cpuinfo_x86 *c)
*
* Affected parts all have no supervisor XSAVE states, meaning that
* the XSAVEC instruction (which works fine) is equivalent.
+ *
+ * Clear the feature flag only on microcode revisions which
+ * don't have the fix.
*/
+ if (x86_cpu_has_min_microcode_rev(erratum_1386_microcode))
+ return;
+
clear_cpu_cap(c, X86_FEATURE_XSAVES);
}

--
2.43.0

--
Regards/Gruss,
Boris.