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Subject[tip: x86/fsgsbase] x86/entry/64: Introduce the FIND_PERCPU_BASE macro
The following commit has been merged into the x86/fsgsbase branch of tip:

Commit-ID: eaad981291ee36efee15a5e515d4598ae94ace07
Gitweb: https://git.kernel.org/tip/eaad981291ee36efee15a5e515d4598ae94ace07
Author: Chang S. Bae <chang.seok.bae@intel.com>
AuthorDate: Thu, 28 May 2020 16:13:56 -04:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 18 Jun 2020 15:47:04 +02:00

x86/entry/64: Introduce the FIND_PERCPU_BASE macro

GSBASE is used to find per-CPU data in the kernel. But when GSBASE is
unknown, the per-CPU base can be found from the per_cpu_offset table with a
CPU NR. The CPU NR is extracted from the limit field of the CPUNODE entry
in GDT, or by the RDPID instruction. This is a prerequisite for using
FSGSBASE in the low level entry code.

Also, add the GAS-compatible RDPID macro as binutils 2.23 do not support
it. Support is added in version 2.27.

[ tglx: Massaged changelog ]

Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1557309753-24073-12-git-send-email-chang.seok.bae@intel.com
Link: https://lkml.kernel.org/r/20200528201402.1708239-11-sashal@kernel.org


---
arch/x86/entry/calling.h | 34 ++++++++++++++++++++++++++++++++++
arch/x86/include/asm/inst.h | 15 +++++++++++++++
2 files changed, 49 insertions(+)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 4208c1e..5c0cbb4 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -6,6 +6,7 @@
#include <asm/percpu.h>
#include <asm/asm-offsets.h>
#include <asm/processor-flags.h>
+#include <asm/inst.h>

/*

@@ -351,3 +352,36 @@ For 32-bit we have the following conventions - kernel is built with
call stackleak_erase
#endif
.endm
+
+#ifdef CONFIG_SMP
+
+/*
+ * CPU/node NR is loaded from the limit (size) field of a special segment
+ * descriptor entry in GDT.
+ */
+.macro LOAD_CPU_AND_NODE_SEG_LIMIT reg:req
+ movq $__CPUNODE_SEG, \reg
+ lsl \reg, \reg
+.endm
+
+/*
+ * Fetch the per-CPU GSBASE value for this processor and put it in @reg.
+ * We normally use %gs for accessing per-CPU data, but we are setting up
+ * %gs here and obviously can not use %gs itself to access per-CPU data.
+ */
+.macro GET_PERCPU_BASE reg:req
+ ALTERNATIVE \
+ "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
+ "RDPID \reg", \
+ X86_FEATURE_RDPID
+ andq $VDSO_CPUNODE_MASK, \reg
+ movq __per_cpu_offset(, \reg, 8), \reg
+.endm
+
+#else
+
+.macro GET_PERCPU_BASE reg:req
+ movq pcpu_unit_offsets(%rip), \reg
+.endm
+
+#endif /* CONFIG_SMP */
diff --git a/arch/x86/include/asm/inst.h b/arch/x86/include/asm/inst.h
index f5a796d..d063841 100644
--- a/arch/x86/include/asm/inst.h
+++ b/arch/x86/include/asm/inst.h
@@ -306,6 +306,21 @@
.endif
MODRM 0xc0 movq_r64_xmm_opd1 movq_r64_xmm_opd2
.endm
+
+.macro RDPID opd
+ REG_TYPE rdpid_opd_type \opd
+ .if rdpid_opd_type == REG_TYPE_R64
+ R64_NUM rdpid_opd \opd
+ .else
+ R32_NUM rdpid_opd \opd
+ .endif
+ .byte 0xf3
+ .if rdpid_opd > 7
+ PFX_REX rdpid_opd 0
+ .endif
+ .byte 0x0f, 0xc7
+ MODRM 0xc0 rdpid_opd 0x7
+.endm
#endif

#endif
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