[PATCH v3 3/4] x86/cpu: Add setcpuid cmdline param

Brendan Jackman
2025-02-18T18:59:54Z

In preparation for adding support to fake out CPU bugs, add a general
facility to force-enable CPU flags.

The flag taints the kernel and the documentation attempts to be clear
that this is highly unsuitable for uses outside of kernel development
and platform experimentation.

The new arg is parsed just like clearcpuid, but instead of leading to
setup_clear_cpu_cap() it leads to setup_force_cpu_cap().

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---

Notes:
I've tested this by booting a nested QEMU
guest on an Intel host, which with setcpuid=svm
will claim that it supports AMD virtualization.

arch/x86/kernel/cpu/common.c | 34 +++++++++++++++++++++++++---------
1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 1241c205b9d0..245807f9351d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1479,12 +1479,12 @@ static void detect_nopl(void)
#endif
}

-static inline void parse_clearcpuid(char *arg)
+static inline void parse_set_clear_cpuid(char *arg, bool set)
{
char *opt;
int taint = 0;

- pr_info("Clearing CPUID bits:");
+ pr_info("%s CPUID bits:", set ? "Force-enabling" : "Clearing");

while (arg) {
bool found __maybe_unused = false;
@@ -1505,7 +1505,10 @@ static inline void parse_clearcpuid(char *arg)
else
pr_cont(" %s", x86_cap_flags[bit]);

- setup_clear_cpu_cap(bit);
+ if (set)
+ setup_force_cpu_cap(bit);
+ else
+ setup_clear_cpu_cap(bit);
taint++;
}
/*
@@ -1523,7 +1526,10 @@ static inline void parse_clearcpuid(char *arg)
continue;

pr_cont(" %s", opt);
- setup_clear_cpu_cap(bit);
+ if (set)
+ setup_force_cpu_cap(bit);
+ else
+ setup_clear_cpu_cap(bit);
taint++;
found = true;
break;
@@ -1579,10 +1585,12 @@ static void __init cpu_parse_early_param(void)
setup_clear_cpu_cap(X86_FEATURE_FRED);

arglen = cmdline_find_option(boot_command_line, "clearcpuid", arg, sizeof(arg));
- if (arglen <= 0)
- return;
+ if (arglen > 0)
+ parse_set_clear_cpuid(arg, false);

- parse_clearcpuid(arg);
+ arglen = cmdline_find_option(boot_command_line, "setcpuid", arg, sizeof(arg));
+ if (arglen > 0)
+ parse_set_clear_cpuid(arg, true);
}

/*
@@ -2014,15 +2022,23 @@ void print_cpu_info(struct cpuinfo_x86 *c)
}

/*
- * clearcpuid= was already parsed in cpu_parse_early_param(). This dummy
- * function prevents it from becoming an environment variable for init.
+ * clearcpuid= and setcpuid= were already parsed in cpu_parse_early_param().
+ * These dummy functions prevent them from becoming an environment variable for
+ * init.
*/
+
static __init int setup_clearcpuid(char *arg)
{
return 1;
}
__setup("clearcpuid=", setup_clearcpuid);

+static __init int setup_setcpuid(char *arg)
+{
+ return 1;
+}
+__setup("setcpuid=", setup_setcpuid);
+
DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
.current_task = &init_task,
.preempt_count = INIT_PREEMPT_COUNT,

--
2.48.1.601.g30ceb7b040-goog