linux-kernel - [PATCH V2 38/41] x86/entry: Remove ASM function paranoid_entry() and paranoid

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Date:   Sun, 26 Sep 2021 23:08:35 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH V2 38/41] x86/entry: Remove ASM function paranoid_entry() and paranoid_exit()

From: Lai Jiangshan <laijs@...ux.alibaba.com>

IST exceptions are changed to use C entry code which uses the C function
ist_paranoid_entry() and ist_paranoid_exit().  The ASM function
paranoid_entry() and paranoid_exit() are useless.

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/entry/entry_64.S | 124 --------------------------------------
 1 file changed, 124 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 63cafeeaf27d..260be3c9da7d 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -815,130 +815,6 @@ SYM_CODE_START(xen_failsafe_callback)
 SYM_CODE_END(xen_failsafe_callback)
 #endif /* CONFIG_XEN_PV */
 
-/*
- * Save all registers in pt_regs. Return GSBASE related information
- * in EBX depending on the availability of the FSGSBASE instructions:
- *
- * FSGSBASE	R/EBX
- *     N        0 -> SWAPGS on exit
- *              1 -> no SWAPGS on exit
- *
- *     Y        GSBASE value at entry, must be restored in paranoid_exit
- */
-SYM_CODE_START_LOCAL(paranoid_entry)
-	UNWIND_HINT_FUNC
-	cld
-
-	/*
-	 * Always stash CR3 in %r14.  This value will be restored,
-	 * verbatim, at exit.  Needed if paranoid_entry interrupted
-	 * another entry that already switched to the user CR3 value
-	 * but has not yet returned to userspace.
-	 *
-	 * This is also why CS (stashed in the "iret frame" by the
-	 * hardware at entry) can not be used: this may be a return
-	 * to kernel code, but with a user CR3 value.
-	 *
-	 * Switching CR3 does not depend on kernel GSBASE so it can
-	 * be done before switching to the kernel GSBASE. This is
-	 * required for FSGSBASE because the kernel GSBASE has to
-	 * be retrieved from a kernel internal table.
-	 */
-	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
-
-	/*
-	 * Handling GSBASE depends on the availability of FSGSBASE.
-	 *
-	 * Without FSGSBASE the kernel enforces that negative GSBASE
-	 * values indicate kernel GSBASE. With FSGSBASE no assumptions
-	 * can be made about the GSBASE value when entering from user
-	 * space.
-	 */
-	ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE
-
-	/*
-	 * Read the current GSBASE and store it in %rbx unconditionally,
-	 * retrieve and set the current CPUs kernel GSBASE. The stored value
-	 * has to be restored in paranoid_exit unconditionally.
-	 *
-	 * The unconditional write to GS base below ensures that no subsequent
-	 * loads based on a mispredicted GS base can happen, therefore no LFENCE
-	 * is needed here.
-	 */
-	SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
-	ret
-
-.Lparanoid_entry_checkgs:
-	/* EBX = 1 -> kernel GSBASE active, no restore required */
-	movl	$1, %ebx
-	/*
-	 * The kernel-enforced convention is a negative GSBASE indicates
-	 * a kernel value. No SWAPGS needed on entry and exit.
-	 */
-	movl	$MSR_GS_BASE, %ecx
-	rdmsr
-	testl	%edx, %edx
-	jns	.Lparanoid_entry_swapgs
-	FENCE_SWAPGS_KERNEL_ENTRY
-	ret
-
-.Lparanoid_entry_swapgs:
-	swapgs
-	FENCE_SWAPGS_USER_ENTRY
-
-	/* EBX = 0 -> SWAPGS required on exit */
-	xorl	%ebx, %ebx
-	ret
-SYM_CODE_END(paranoid_entry)
-
-/*
- * "Paranoid" exit path from exception stack.  This is invoked
- * only on return from IST interrupts that came from kernel space.
- *
- * We may be returning to very strange contexts (e.g. very early
- * in syscall entry), so checking for preemption here would
- * be complicated.  Fortunately, there's no good reason to try
- * to handle preemption here.
- *
- * R/EBX contains the GSBASE related information depending on the
- * availability of the FSGSBASE instructions:
- *
- * FSGSBASE	R/EBX
- *     N        0 -> SWAPGS on exit
- *              1 -> no SWAPGS on exit
- *
- *     Y        User space GSBASE, must be restored unconditionally
- */
-SYM_CODE_START_LOCAL(paranoid_exit)
-	UNWIND_HINT_REGS offset=8
-	/*
-	 * The order of operations is important. RESTORE_CR3 requires
-	 * kernel GSBASE.
-	 *
-	 * NB to anyone to try to optimize this code: this code does
-	 * not execute at all for exceptions from user mode. Those
-	 * exceptions go through error_exit instead.
-	 */
-	RESTORE_CR3	scratch_reg=%rax save_reg=%r14
-
-	/* Handle the three GSBASE cases */
-	ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE
-
-	/* With FSGSBASE enabled, unconditionally restore GSBASE */
-	wrgsbase	%rbx
-	ret
-
-.Lparanoid_exit_checkgs:
-	/* On non-FSGSBASE systems, conditionally do SWAPGS */
-	testl		%ebx, %ebx
-	jnz		.Lparanoid_exit_done
-
-	/* We are returning to a context with user GSBASE */
-	swapgs
-.Lparanoid_exit_done:
-	ret
-SYM_CODE_END(paranoid_exit)
-
 SYM_CODE_START_LOCAL(error_return)
 	UNWIND_HINT_REGS
 	DEBUG_ENTRY_ASSERT_IRQS_OFF
-- 
2.19.1.6.gb485710b