linux-kernel - [PATCH v2] x86/entry/64/paravirt: Use paravirt-safe macro to access eflags

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Date:   Tue, 28 Nov 2017 10:28:25 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, jgross@...e.com, luto@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: [PATCH v2] x86/entry/64/paravirt: Use paravirt-safe macro to access eflags

Commit 1d3e53e8624a ("x86/entry/64: Refactor IRQ stacks and make
them NMI-safe") added DEBUG_ENTRY_ASSERT_IRQS_OFF macro that acceses
eflags using 'pushfq' instruction when testing for IF bit. On PV Xen
guests looking at IF flag directly will always see it set, resulting
in 'ud2'.

Introduce SAVE_FLAGS() macro that will use appropriate save_fl pv op
when running paravirt.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
---
V2:
* Preserve %rax in DEBUG_ENTRY_ASSERT_IRQS_OFF
* Return (pop) %rax in SAVE_FLAGS for !CONFIG_PARAVIRT (irqflags.h)

 arch/x86/entry/entry_64.S        |    7 ++++---
 arch/x86/include/asm/irqflags.h  |    3 +++
 arch/x86/include/asm/paravirt.h  |    9 +++++++++
 arch/x86/kernel/asm-offsets_64.c |    3 +++
 4 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f81d50d..c208dc1 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -466,12 +466,13 @@ END(irq_entries_start)
 
 .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
 #ifdef CONFIG_DEBUG_ENTRY
-	pushfq
-	testl $X86_EFLAGS_IF, (%rsp)
+	pushq %rax
+	SAVE_FLAGS(CLBR_ANY)
+	testl $X86_EFLAGS_IF, %eax
 	jz .Lokay_\@
 	ud2
 .Lokay_\@:
-	addq $8, %rsp
+	popq %rax
 #endif
 .endm
 
diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
index c8ef23f..89f0895 100644
--- a/arch/x86/include/asm/irqflags.h
+++ b/arch/x86/include/asm/irqflags.h
@@ -142,6 +142,9 @@ static inline notrace unsigned long arch_local_irq_save(void)
 	swapgs;					\
 	sysretl
 
+#ifdef CONFIG_DEBUG_ENTRY
+#define SAVE_FLAGS(x)		pushfq; popq %rax
+#endif
 #else
 #define INTERRUPT_RETURN		iret
 #define ENABLE_INTERRUPTS_SYSEXIT	sti; sysexit
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 283efca..892df37 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -927,6 +927,15 @@ static inline notrace unsigned long arch_local_irq_save(void)
 	PARA_SITE(PARA_PATCH(pv_cpu_ops, PV_CPU_usergs_sysret64),	\
 		  CLBR_NONE,						\
 		  jmp PARA_INDIRECT(pv_cpu_ops+PV_CPU_usergs_sysret64))
+
+#ifdef CONFIG_DEBUG_ENTRY
+#define SAVE_FLAGS(clobbers)                                        \
+	PARA_SITE(PARA_PATCH(pv_irq_ops, PV_IRQ_save_fl), clobbers, \
+		  PV_SAVE_REGS(clobbers | CLBR_CALLEE_SAVE);        \
+		  call PARA_INDIRECT(pv_irq_ops+PV_IRQ_save_fl);    \
+		  PV_RESTORE_REGS(clobbers | CLBR_CALLEE_SAVE);)
+#endif
+
 #endif	/* CONFIG_X86_32 */
 
 #endif /* __ASSEMBLY__ */
diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
index 630212f..e3a5175 100644
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -23,6 +23,9 @@ int main(void)
 #ifdef CONFIG_PARAVIRT
 	OFFSET(PV_CPU_usergs_sysret64, pv_cpu_ops, usergs_sysret64);
 	OFFSET(PV_CPU_swapgs, pv_cpu_ops, swapgs);
+#ifdef CONFIG_DEBUG_ENTRY
+	OFFSET(PV_IRQ_save_fl, pv_irq_ops, save_fl);
+#endif
 	BLANK();
 #endif
 
-- 
1.7.1