linux-kernel - [RFC][PATCH 18/22] x86,kvm: Remove .fixup usage

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Message-ID: <20211104165525.588957687@infradead.org>
Date:   Thu, 04 Nov 2021 17:47:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        jpoimboe@...hat.com, mark.rutland@....com, dvyukov@...gle.com,
        seanjc@...gle.com, pbonzini@...hat.com, mbenes@...e.cz
Subject: [RFC][PATCH 18/22] x86,kvm: Remove .fixup usage

KVM instruction emulation has a gnarly hack where the .fixup does a
return, luckily exceptions can easily emulate a return so create a new
extable type to fully do the FASTOP magic.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/extable_fixup_types.h |    1 +
 arch/x86/kvm/emulate.c                     |   14 +++-----------
 arch/x86/mm/extable.c                      |   11 +++++++++++
 3 files changed, 15 insertions(+), 11 deletions(-)

--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -40,5 +40,6 @@
 #define	EX_TYPE_ONE_REG			(EX_TYPE_IMM_REG | EX_TYPE_IMM(1))
 
 #define	EX_TYPE_FAULT_SGX		18
+#define	EX_TYPE_KVM_FASTOP		19
 
 #endif
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -344,7 +344,7 @@ static int fastop(struct x86_emulate_ctx
 	__FOP_RET(#op "_" #dst)
 
 #define FOP1EEX(op,  dst) \
-	FOP1E(op, dst) _ASM_EXTABLE(10b, kvm_fastop_exception)
+	FOP1E(op, dst) _ASM_EXTABLE_TYPE(10b, 10b, EX_TYPE_KVM_FASTOP)
 
 #define FASTOP1(op) \
 	FOP_START(op) \
@@ -434,10 +434,6 @@ static int fastop(struct x86_emulate_ctx
 	#op " %al \n\t" \
 	__FOP_RET(#op)
 
-asm(".pushsection .fixup, \"ax\"\n"
-    "kvm_fastop_exception: xor %esi, %esi; ret\n"
-    ".popsection");
-
 FOP_START(setcc)
 FOP_SETCC(seto)
 FOP_SETCC(setno)
@@ -473,12 +469,8 @@ FOP_END;
  \
 	asm volatile("1:" insn "\n" \
 	             "2:\n" \
-	             ".pushsection .fixup, \"ax\"\n" \
-	             "3: movl $1, %[_fault]\n" \
-	             "   jmp  2b\n" \
-	             ".popsection\n" \
-	             _ASM_EXTABLE(1b, 3b) \
-	             : [_fault] "+qm"(_fault) inoutclob ); \
+		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_ONE_REG, %[_fault]) \
+	             : [_fault] "+r"(_fault) inoutclob ); \
  \
 	_fault ? X86EMUL_UNHANDLEABLE : X86EMUL_CONTINUE; \
 })
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -10,6 +10,7 @@
 #include <asm/traps.h>
 #include <asm/kdebug.h>
 #include <asm/sgx.h>
+#include <asm/text-patching.h>
 
 static inline unsigned long *pt_regs_nr(struct pt_regs *regs, int nr)
 {
@@ -154,6 +155,14 @@ static bool ex_handler_sgx(const struct
 	return ex_handler_default(fixup, regs);
 }
 
+static bool ex_handler_kvm_fastop(const struct exception_table_entry *fixup,
+				  struct pt_regs *regs)
+{
+	regs->si = 0;
+	int3_emulate_ret(regs);
+	return true;
+}
+
 #define EX_TYPE_MASK	0x000000FF
 #define EX_REG_MASK	0x00000F00
 #define EX_FLAG_MASK	0x0000F000
@@ -231,6 +240,8 @@ int fixup_exception(struct pt_regs *regs
 		return ex_handler_imm_reg(e, regs, reg, imm);
 	case EX_TYPE_FAULT_SGX:
 		return ex_handler_sgx(e, regs, trapnr);
+	case EX_TYPE_KVM_FASTOP:
+		return ex_handler_kvm_fastop(e, regs);
 	}
 	BUG();
 }