Linux Virtualization - Mar 2023 - [PATCH v2] x86/paravirt: convert simple paravirt functions to asm

All functions referenced via __PV_IS_CALLEE_SAVE() need to be assembler
functions, as those functions calls are hidden from gcc. In case the
kernel is compiled with "-fzero-call-used-regs" the compiler will
clobber caller-saved registers at the end of C functions, which will
result in unexpectedly zeroed registers at the call site of the
related paravirt functions.

Replace the C functions with DEFINE_PARAVIRT_ASM() constructs using
the same instructions as the related paravirt calls in the
PVOP_ALT_[V]CALLEE*() macros.

Signed-off-by: Juergen Gross <jgross at suse.com>
---
V2:
- use noinstr section (Peter Zijlstra, Andrew Cooper)
---
 arch/x86/include/asm/paravirt_types.h |  8 +++++++-
 arch/x86/kernel/paravirt.c            | 27 ++++++---------------------
 2 files changed, 13 insertions(+), 22 deletions(-)

diff --git a/arch/x86/include/asm/paravirt_types.h
b/arch/x86/include/asm/paravirt_types.h
index 8c1da419260f..49f5c6955229 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -562,8 +562,14 @@ void paravirt_flush_lazy_mmu(void);
 
 void _paravirt_nop(void);
 void paravirt_BUG(void);
-u64 _paravirt_ident_64(u64);
 unsigned long paravirt_ret0(void);
+#ifdef CONFIG_PARAVIRT_XXL
+u64 _paravirt_ident_64(u64);
+unsigned long pv_native_save_fl(void);
+void pv_native_irq_disable(void);
+void pv_native_irq_enable(void);
+unsigned long pv_native_read_cr2(void);
+#endif
 
 #define paravirt_nop	((void *)_paravirt_nop)
 
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 42e182868873..5ec2071570a4 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -64,11 +64,11 @@ static unsigned paravirt_patch_call(void *insn_buff, const
void *target,
 }
 
 #ifdef CONFIG_PARAVIRT_XXL
-/* identity function, which can be inlined */
-u64 notrace _paravirt_ident_64(u64 x)
-{
-	return x;
-}
+DEFINE_PARAVIRT_ASM(_paravirt_ident_64, "mov %rdi, %rax", .text);
+DEFINE_PARAVIRT_ASM(pv_native_save_fl, "pushf; pop %rax",
.noinstr.text);
+DEFINE_PARAVIRT_ASM(pv_native_irq_disable, "cli", .noinstr.text);
+DEFINE_PARAVIRT_ASM(pv_native_irq_enable, "sti", .noinstr.text);
+DEFINE_PARAVIRT_ASM(pv_native_read_cr2, "mov %cr2, %rax",
.noinstr.text);
 #endif
 
 DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
@@ -197,11 +197,6 @@ void paravirt_end_context_switch(struct task_struct *next)
 		arch_enter_lazy_mmu_mode();
 }
 
-static noinstr unsigned long pv_native_read_cr2(void)
-{
-	return native_read_cr2();
-}
-
 static noinstr void pv_native_write_cr2(unsigned long val)
 {
 	native_write_cr2(val);
@@ -222,16 +217,6 @@ noinstr void pv_native_wbinvd(void)
 	native_wbinvd();
 }
 
-static noinstr void pv_native_irq_enable(void)
-{
-	native_irq_enable();
-}
-
-static noinstr void pv_native_irq_disable(void)
-{
-	native_irq_disable();
-}
-
 static noinstr void pv_native_safe_halt(void)
 {
 	native_safe_halt();
@@ -298,7 +283,7 @@ struct paravirt_patch_template pv_ops = {
 	.cpu.end_context_switch		= paravirt_nop,
 
 	/* Irq ops. */
-	.irq.save_fl		= __PV_IS_CALLEE_SAVE(native_save_fl),
+	.irq.save_fl		= __PV_IS_CALLEE_SAVE(pv_native_save_fl),
 	.irq.irq_disable	= __PV_IS_CALLEE_SAVE(pv_native_irq_disable),
 	.irq.irq_enable		= __PV_IS_CALLEE_SAVE(pv_native_irq_enable),
 	.irq.safe_halt		= pv_native_safe_halt,
-- 
2.35.3