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Date:   Mon, 16 Nov 2020 14:04:13 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     =?iso-8859-1?Q?J=FCrgen_Gro=DF?= <jgross@suse.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        X86 ML <x86@kernel.org>
Subject: Re: WARNING: can't access registers at asm_common_interrupt
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On Mon, Nov 16, 2020 at 12:56:32PM +0100, J=FCrgen Gro=DF wrote:
> > > > > > > static inline notrace unsigned long arch_local_save_flags(voi=
d)
> > > > > > > {
> > > > > > >     PVOP_CALL_ARGS;
> > > > > > >     PVOP_TEST_NULL(irq.save_fl);
> > > > > > >     asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CAL=
L),
> > > > > > >                                     "PUSHF; POP _ASM_AX",
> > > > > > >                                     X86_FEATURE_NATIVE)
>=20
> I am wondering whether we really want a new feature (basically "not
> XENPV). We could use ~X86_FEATURE_XENPV and teach apply_alternatives()
> to understand negated features (yes, this limits the number of features
> to 32767, but I don't think this is a real problem for quite some time).
>=20
> Thoughts?

I went with the simple thing for now... If we ever want/need another
negative alternative I suppose we can do as you suggest...

I was still poking at objtool to actually dtrt though..

---
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpuf=
eatures.h
index dad350d42ecf..cc88f358bac5 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -237,6 +237,7 @@
 #define X86_FEATURE_VMCALL		( 8*32+18) /* "" Hypervisor supports the VMCAL=
L instruction */
 #define X86_FEATURE_VMW_VMMCALL		( 8*32+19) /* "" VMware prefers VMMCALL h=
ypercall instruction */
 #define X86_FEATURE_SEV_ES		( 8*32+20) /* AMD Secure Encrypted Virtualizat=
ion - Encrypted State */
+#define X86_FEATURE_NOT_XENPV		( 8*32+21) /* "" inverse of X86_FEATURE_XEN=
PV */
=20
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (EBX), word 9 */
 #define X86_FEATURE_FSGSBASE		( 9*32+ 0) /* RDFSBASE, WRFSBASE, RDGSBASE, =
WRGSBASE instructions*/
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravir=
t.h
index d25cc6830e89..e2a9d3e6b7ad 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -645,22 +645,56 @@ bool __raw_callee_save___native_vcpu_is_preempted(lon=
g cpu);
 #ifdef CONFIG_PARAVIRT_XXL
 static inline notrace unsigned long arch_local_save_flags(void)
 {
-	return PVOP_CALLEE0(unsigned long, irq.save_fl);
+	PVOP_CALL_ARGS;
+	PVOP_TEST_NULL(irq.save_fl);
+	asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
+					"pushf; pop %%" _ASM_AX ";",
+					X86_FEATURE_NOT_XENPV)
+			    : PVOP_CALLEE_CLOBBERS, ASM_CALL_CONSTRAINT
+			    : paravirt_type(irq.save_fl.func),
+			      paravirt_clobber(CLBR_RET_REG)
+			    : "memory", "cc");
+	return __eax;
 }
=20
 static inline notrace void arch_local_irq_restore(unsigned long f)
 {
-	PVOP_VCALLEE1(irq.restore_fl, f);
+	PVOP_CALL_ARGS;
+	PVOP_TEST_NULL(irq.restore_fl);
+	asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
+					"push %%" _ASM_ARG1 "; popf;",
+					X86_FEATURE_NOT_XENPV)
+			    : PVOP_VCALLEE_CLOBBERS, ASM_CALL_CONSTRAINT
+			    : paravirt_type(irq.restore_fl.func),
+			      paravirt_clobber(CLBR_RET_REG),
+			      PVOP_CALL_ARG1(f)
+			    : "memory", "cc");
 }
=20
 static inline notrace void arch_local_irq_disable(void)
 {
-	PVOP_VCALLEE0(irq.irq_disable);
+	PVOP_CALL_ARGS;
+	PVOP_TEST_NULL(irq.irq_disable);
+	asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
+					"cli;",
+					X86_FEATURE_NOT_XENPV)
+			    : PVOP_VCALLEE_CLOBBERS, ASM_CALL_CONSTRAINT
+			    : paravirt_type(irq.irq_disable.func),
+			      paravirt_clobber(CLBR_RET_REG)
+			    : "memory", "cc");
 }
=20
 static inline notrace void arch_local_irq_enable(void)
 {
-	PVOP_VCALLEE0(irq.irq_enable);
+	PVOP_CALL_ARGS;
+	PVOP_TEST_NULL(irq.irq_enable);
+	asm_inline volatile(ALTERNATIVE(paravirt_alt(PARAVIRT_CALL),
+					"sti;",
+					X86_FEATURE_NOT_XENPV)
+			    : PVOP_VCALLEE_CLOBBERS, ASM_CALL_CONSTRAINT
+			    : paravirt_type(irq.irq_enable.func),
+			      paravirt_clobber(CLBR_RET_REG)
+			    : "memory", "cc");
 }
=20
 static inline notrace unsigned long arch_local_irq_save(void)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 2400ad62f330..5bd8f5503652 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -723,6 +723,19 @@ void __init alternative_instructions(void)
 	 * patching.
 	 */
=20
+	if (!boot_cpu_has(X86_FEATURE_XENPV))
+		setup_force_cpu_cap(X86_FEATURE_NOT_XENPV);
+
+	/*
+	 * First patch paravirt functions, such that we overwrite the indirect
+	 * call with the direct call.
+	 */
+	apply_paravirt(__parainstructions, __parainstructions_end);
+
+	/*
+	 * Then patch alternatives, such that those paravirt calls that are in
+	 * alternatives can be overwritten by their immediate fragments.
+	 */
 	apply_alternatives(__alt_instructions, __alt_instructions_end);
=20
 #ifdef CONFIG_SMP
@@ -741,8 +754,6 @@ void __init alternative_instructions(void)
 	}
 #endif
=20
-	apply_paravirt(__parainstructions, __parainstructions_end);
-
 	restart_nmi();
 	alternatives_patched =3D 1;
 }
diff --git a/arch/x86/kernel/paravirt_patch.c b/arch/x86/kernel/paravirt_pa=
tch.c
index ace6e334cb39..867498db53ad 100644
--- a/arch/x86/kernel/paravirt_patch.c
+++ b/arch/x86/kernel/paravirt_patch.c
@@ -33,13 +33,9 @@ struct patch_xxl {
 };
=20
 static const struct patch_xxl patch_data_xxl =3D {
-	.irq_irq_disable	=3D { 0xfa },		// cli
-	.irq_irq_enable		=3D { 0xfb },		// sti
-	.irq_save_fl		=3D { 0x9c, 0x58 },	// pushf; pop %[re]ax
 	.mmu_read_cr2		=3D { 0x0f, 0x20, 0xd0 },	// mov %cr2, %[re]ax
 	.mmu_read_cr3		=3D { 0x0f, 0x20, 0xd8 },	// mov %cr3, %[re]ax
 	.mmu_write_cr3		=3D { 0x0f, 0x22, 0xdf },	// mov %rdi, %cr3
-	.irq_restore_fl		=3D { 0x57, 0x9d },	// push %rdi; popfq
 	.cpu_wbinvd		=3D { 0x0f, 0x09 },	// wbinvd
 	.cpu_usergs_sysret64	=3D { 0x0f, 0x01, 0xf8,
 				    0x48, 0x0f, 0x07 },	// swapgs; sysretq
@@ -76,11 +72,6 @@ unsigned int native_patch(u8 type, void *insn_buff, unsi=
gned long addr,
 	switch (type) {
=20
 #ifdef CONFIG_PARAVIRT_XXL
-	PATCH_CASE(irq, restore_fl, xxl, insn_buff, len);
-	PATCH_CASE(irq, save_fl, xxl, insn_buff, len);
-	PATCH_CASE(irq, irq_enable, xxl, insn_buff, len);
-	PATCH_CASE(irq, irq_disable, xxl, insn_buff, len);
-
 	PATCH_CASE(mmu, read_cr2, xxl, insn_buff, len);
 	PATCH_CASE(mmu, read_cr3, xxl, insn_buff, len);
 	PATCH_CASE(mmu, write_cr3, xxl, insn_buff, len);

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