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Subject[PATCH] "noht" disable HyperThreading
It's good that 2.4.19-pre on Xeon now defaults to HyperThreading on,
but currently there's no way to disable it. Wouldn't it be prudent
to support a "noht" boot arg? (Why retain obscure "acpismp=force"?
because it's conceivable that it could help some non-HT machine to
boot, which fails to boot otherwise.)

Hugh

--- 2.4.19-pre8/Documentation/kernel-parameters.txt Mon Feb 25 19:37:51 2002
+++ linux/Documentation/kernel-parameters.txt Fri May 10 14:16:58 2002
@@ -68,6 +68,8 @@
53c7xx= [HW,SCSI] Amiga SCSI controllers.

acpi= [HW,ACPI] Advanced Configuration and Power Interface
+
+ acpismp=force [IA-32] Early setup parse and use ACPI SMP table.

ad1816= [HW,SOUND]

@@ -378,6 +380,8 @@
nohlt [BUGS=ARM]

no-hlt [BUGS=ix86]
+
+ noht [SMP,IA-32] Disables P4 Xeon(tm) HyperThreading.

noisapnp [ISAPNP] Disables ISA PnP code.

--- 2.4.19-pre8/arch/i386/kernel/setup.c Fri May 3 12:15:08 2002
+++ linux/arch/i386/kernel/setup.c Fri May 10 14:23:07 2002
@@ -166,6 +166,7 @@

static int disable_x86_serial_nr __initdata = 1;
static int disable_x86_fxsr __initdata = 0;
+static int disable_x86_ht __initdata = 0;

int enable_acpi_smp_table;

@@ -727,7 +728,7 @@
} /* setup_memory_region */


-static void __init parse_mem_cmdline (char ** cmdline_p)
+static void __init parse_cmdline_early (char ** cmdline_p)
{
char c = ' ', *to = command_line, *from = COMMAND_LINE;
int len = 0;
@@ -738,6 +739,8 @@
saved_command_line[COMMAND_LINE_SIZE-1] = '\0';

for (;;) {
+ if (c != ' ')
+ goto nextchar;
/*
* "mem=nopentium" disables the 4MB page tables.
* "mem=XXX[kKmM]" defines a memory region from HIGH_MEM
@@ -745,7 +748,7 @@
* "mem=XXX[KkmM]@XXX[KkmM]" defines a memory region from
* <start> to <start>+<mem>, overriding the bios size.
*/
- if (c == ' ' && !memcmp(from, "mem=", 4)) {
+ if (!memcmp(from, "mem=", 4)) {
if (to != command_line)
to--;
if (!memcmp(from+4, "nopentium", 9)) {
@@ -774,17 +777,23 @@
}
}
}
- /* acpismp=force forces parsing and use of the ACPI SMP table */
- if (c == ' ' && !memcmp(from, "acpismp=force", 13))
- enable_acpi_smp_table = 1;
+
+ /* "noht" disables HyperThreading (2 logical cpus per Xeon) */
+ else if (!memcmp(from, "noht", 4))
+ disable_x86_ht = 1;
+
+ /* "acpismp=force" forces parsing and use of the ACPI SMP table */
+ else if (!memcmp(from, "acpismp=force", 13))
+ enable_acpi_smp_table = 1;
+
/*
* highmem=size forces highmem to be exactly 'size' bytes.
* This works even on boxes that have no highmem otherwise.
* This also works to reduce highmem size on bigger boxes.
*/
- if (c == ' ' && !memcmp(from, "highmem=", 8))
+ else if (!memcmp(from, "highmem=", 8))
highmem_pages = memparse(from+8, &from) >> PAGE_SHIFT;
-
+nextchar:
c = *(from++);
if (!c)
break;
@@ -841,7 +850,7 @@
data_resource.start = virt_to_bus(&_etext);
data_resource.end = virt_to_bus(&_edata)-1;

- parse_mem_cmdline(cmdline_p);
+ parse_cmdline_early(cmdline_p);

#define PFN_UP(x) (((x) + PAGE_SIZE-1) >> PAGE_SHIFT)
#define PFN_DOWN(x) ((x) >> PAGE_SHIFT)
@@ -1025,6 +1034,17 @@
}
#endif

+ /*
+ * If enable_acpi_smp_table and HT feature present, acpitable.c
+ * will find all logical cpus despite disable_x86_ht: so if both
+ * "noht" and "acpismp=force" are specified, let "noht" override
+ * "acpismp=force" cleanly. Why retain "acpismp=force"? because
+ * parsing ACPI SMP table might prove useful on some non-HT cpu.
+ */
+ if (disable_x86_ht) {
+ clear_bit(X86_FEATURE_HT, &boot_cpu_data.x86_capability[0]);
+ enable_acpi_smp_table = 0;
+ }
if (test_bit(X86_FEATURE_HT, &boot_cpu_data.x86_capability[0]))
enable_acpi_smp_table = 1;

@@ -2267,7 +2287,7 @@
strcpy(c->x86_model_id, p);

#ifdef CONFIG_SMP
- if (test_bit(X86_FEATURE_HT, &c->x86_capability)) {
+ if (test_bit(X86_FEATURE_HT, &c->x86_capability) && !disable_x86_ht) {
extern int phys_proc_id[NR_CPUS];

u32 eax, ebx, ecx, edx;
@@ -2717,6 +2737,10 @@
if ( tsc_disable )
clear_bit(X86_FEATURE_TSC, &c->x86_capability);
#endif
+
+ /* HT disabled? */
+ if (disable_x86_ht)
+ clear_bit(X86_FEATURE_HT, &c->x86_capability);

/* FXSR disabled? */
if (disable_x86_fxsr) {
-
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