[PATCH v4 0/2] Add support for cmpxchg16b emulation

[PATCH v4 0/2] Add support for cmpxchg16b emulation

[Sairaj Kodilkar]

This series adds 128-bit compare-exchange support needed for KVM to
emulate guest cmpxchg16b instructions.

The AMD IOMMU driver updates 256-bit device table entries with two
128-bit cmpxchg operations.  For hardware-accelerated vIOMMU, QEMU traps
those DTE accesses to install nested page tables [1].  KVM must emulate
guest cmpxchg16b on the trapped MMIO regions; without that, DTE access
emulation fails.

Patch 1: extends the x86 user CMPXCHG helpers with
__try_cmpxchg128_user_asm() (cmpxchg16b on x86-64), wired into
unsafe_try_cmpxchg_user().

Patch 2: extends KVM CMPXCHG8B emulation to 16-byte operands (REX.W=1) and
uses the new helper for atomic 16-byte compare-exchange on guest RAM via
emulator_cmpxchg_emulated().

You can find the KUT to test this series at [2].

[1] https://github.com/AMDESE/qemu-iommu/blob/wip/for_iommufd_hw_queue-v8_amd_viommu_20260106/hw/i386/amd_viommu.c#L517
[2] https://lore.kernel.org/kvm/20260706062153.346-1-sarunkod@amd.com/T/#u

Changes since v3:
https://lore.kernel.org/all/20260826070004.8100-1-sarunkod@amd.com/
Patch1: Split the old value across RAX and RDX using "+a" and "+b" instead of
	using "+A" [Dave]
Patch 2: Use __c instead of ctx in cmpxchg macro [Sashiko]

Changes since v2:
https://lore.kernel.org/all/20260706063035.1139-1-sarunkod@amd.com/
Patch 1: Trigger build failure if __try_cmpxchg128_user_asm is called on 32 bit
	 machines [Sean]

Sairaj Kodilkar (2):
  x86/uaccess: Extend CMPXCHG user helpers to 128-bit operands
  KVM: x86: Add support for cmpxchg16b emulation

 arch/x86/include/asm/uaccess.h | 64 +++++++++++++++++++++++++++++++++-
 arch/x86/kvm/emulate.c         | 50 +++++++++++++++++---------
 arch/x86/kvm/kvm_emulate.h     |  6 ++++
 arch/x86/kvm/x86.c             |  9 ++++-
 4 files changed, 111 insertions(+), 18 deletions(-)

base-commit: 45c13f3f9e3bb15fd89ff2864c6f627a3b4b4229
-- 
2.34.1

[PATCH v4 1/2] x86/uaccess: Extend CMPXCHG user helpers to 128-bit operands

[Sairaj Kodilkar]

Extend the existing user CMPXCHG helpers to support 16-byte operands on
x86-64, using LOCK_PREFIX "cmpxchg16b".  This mirrors the existing
__try_cmpxchg64_user_asm() / cmpxchg8b path provided for 32-bit kernels,
where KVM needs an atomic compare-exchange wider than the generic
cmpxchg helper can provide.

On 32-bit kernels, stub the helper to generate build failure because
cmpxchg16b requires 64-bit GPRs and is not available.

KVM uses this to atomically emulate guest cmpxchg16b on guest RAM mapped
via userspace addresses.

Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
---
 arch/x86/include/asm/uaccess.h | 64 +++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 3a0dd3c2b233..6e10a8a3ba4d 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -407,6 +407,27 @@ do {									\
 	if (unlikely(!success))						\
 		*_old = __old;						\
 	likely(success);					})
+#else // !CONFIG_X86_32
+#define __try_cmpxchg128_user_asm(_ptr, _pold, _new, label)	({	\
+	bool success;							\
+	__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);		\
+	u64 __old_low = (u64)*_old;					\
+	u64 __old_high = (u64)(*_old >> 64);				\
+	__typeof__(*(_ptr)) __new = (_new);				\
+	asm_goto_output("\n"						\
+		     "1: " LOCK_PREFIX "cmpxchg16b %[ptr]\n"		\
+		     _ASM_EXTABLE_UA(1b, %l[label])			\
+		     : "=@ccz" (success),				\
+		       "+a" (__old_low),				\
+		       "+d" (__old_high),				\
+		       [ptr] "+m" (*_ptr)				\
+		     : "b" ((u64)__new),				\
+		       "c" ((u64)((u128)__new >> 64))			\
+		     : "memory"						\
+		     : label);						\
+	if (unlikely(!success))						\
+		*_old = ((u128)__old_high << 64) | __old_low;		\
+	likely(success);					})
 #endif // CONFIG_X86_32
 #else  // !CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
 #define __try_cmpxchg_user_asm(itype, ltype, _ptr, _pold, _new, label)	({ \
@@ -463,6 +484,32 @@ do {									\
 	if (unlikely(!__result))					\
 		*_old = __old;						\
 	likely(__result);					})
+#else //!CONFIG_X86_32
+#define __try_cmpxchg128_user_asm(_ptr, _pold, _new, label)	({	\
+	int __result;							\
+	__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);		\
+	u64 __old_low = (u64)*_old;					\
+	u64 __old_high = (u64)(*_old >> 64);				\
+	__typeof__(*(_ptr)) __new = (_new);				\
+	asm volatile("\n"						\
+		     "1: " LOCK_PREFIX "cmpxchg16b %[ptr]\n"		\
+		     "mov $0, %[result]\n\t"				\
+		     "setz %b[result]\n"				\
+		     "2:\n"						\
+		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
+					   %[result])			\
+		     : [result] "=q" (__result),			\
+		       "+a" (__old_low),				\
+		       "+d" (__old_high),				\
+		       [ptr] "+m" (*_ptr)				\
+		     : "b" ((u64)__new),				\
+		       "c" ((u64)((u128)__new >> 64))			\
+		     : "memory", "cc");					\
+	if (unlikely(__result < 0))					\
+		goto label;						\
+	if (unlikely(!__result))					\
+		*_old = ((u128)__old_high << 64) | __old_low;		\
+	likely(__result);					})
 #endif // CONFIG_X86_32
 #endif // CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
 
@@ -551,11 +598,18 @@ do {										\
 
 extern void __try_cmpxchg_user_wrong_size(void);
 
-#ifndef CONFIG_X86_32
+#if defined(CONFIG_X86_32) || !defined(X86_FEATURE_CX16)
+/*
+ * Always fail on 32 bit arch or 64 arch without 128 bit cmpxchg support
+ */
+#define __try_cmpxchg128_user_asm(_ptr, _pold, _new, label) ({ BUILD_BUG_ON(1); 0; })
+#else
 #define __try_cmpxchg64_user_asm(_ptr, _oldp, _nval, _label)		\
 	__try_cmpxchg_user_asm("q", "r", (_ptr), (_oldp), (_nval), _label)
+
 #endif
 
+
 /*
  * Force the pointer to u<size> to match the size expected by the asm helper.
  * clang/LLVM compiles all cases and only discards the unused paths after
@@ -580,6 +634,14 @@ extern void __try_cmpxchg_user_wrong_size(void);
 	case 8:	__ret = __try_cmpxchg64_user_asm((__force u64 *)(_ptr), (_oldp),\
 						 (_nval), _label);		\
 		break;								\
+	case 16:								\
+		if (boot_cpu_has(X86_FEATURE_CX16))				\
+			__ret = __try_cmpxchg128_user_asm(			\
+					(__force u128 *)(_ptr),			\
+					(_oldp), (_nval), _label);		\
+		else								\
+			__ret = 0;						\
+		break;								\
 	default: __try_cmpxchg_user_wrong_size();				\
 	}									\
 	__ret;						})
-- 
2.34.1

Re: [PATCH v4 1/2] x86/uaccess: Extend CMPXCHG user helpers to 128-bit operands

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] When the host CPU lacks CX16 support, the 16-byte fallback returns a mismatch without updating the old value pointer, violating the cmpxchg API contract and causing infinite retry loops.
--

commit 5f4dae7c1efaa8622accc402392f8d8a532481b4
Author: Sairaj Kodilkar <sarunkod@amd.com>

x86/uaccess: Extend CMPXCHG user helpers to 128-bit operands

This commit extends the user CMPXCHG helpers to support 16-byte operands
on x86-64 using cmpxchg16b. KVM uses this to atomically emulate guest
cmpxchg16b on guest RAM mapped via userspace addresses.

> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
[ ... ]
> @@ -580,6 +634,14 @@ extern void __try_cmpxchg_user_wrong_size(void);
>  	case 8:	__ret = __try_cmpxchg64_user_asm((__force u64 *)(_ptr), (_oldp),\
>  						 (_nval), _label);		\
>  		break;								\
> +	case 16:								\
> +		if (boot_cpu_has(X86_FEATURE_CX16))				\
> +			__ret = __try_cmpxchg128_user_asm(			\
> +					(__force u128 *)(_ptr),			\
> +					(_oldp), (_nval), _label);		\
> +		else								\
> +			__ret = 0;						\

[Severity: High]
Does setting __ret to 0 here on hardware without CX16 support violate the
cmpxchg API contract?

When boot_cpu_has(X86_FEATURE_CX16) is false, it returns 0 (mismatch) but
fails to fetch the actual memory value and write it into _oldp.

If a guest executes cmpxchg16b on an MMIO address (forcing KVM emulation),
KVM's emulator decodes it and invokes unsafe_try_cmpxchg_user. Because
it returns 0 without updating the pointer, x86_emulate_insn skips
writeback_registers() and returns EMULATION_OK. 

The guest registers remain unchanged while EFLAGS shows failure (ZF=0).

Can this cause a denial of service where the guest endlessly retries the
instruction with the same stale registers?

Should this instead use goto _label; to cleanly abort the unsupported
operation, which would cause KVM to properly abort the emulation?

> +		break;								\
>  	default: __try_cmpxchg_user_wrong_size();				\
>  	}									\
>  	__ret;						})

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908074739.10748-1-sarunkod@amd.com?part=1

[PATCH v4 2/2] KVM: x86: Add support for cmpxchg16b emulation

[Sairaj Kodilkar]

AMD and Intel both provides support for 128 bit cmpxchg operands using
cmpxchg8b/cmpxchg16b instructions (opcode 0FC7). However, kvm does not
support emulating cmpxchg16b (i.e when destination memory is 128 bit and
REX.W = 1) which causes emulation failure when QEMU guest performs a
cmpxchg16b on a memory region setup as a IO.

This failure is seen on the AMD IOMMU driver which writes 256-bit device
table entries with two 128-bit cmpxchg operations. For guests using
hardware-accelerated vIOMMU, QEMU traps device table accesses to set up
nested page tables (see [1]).  Without 128-bit cmpxchg emulation, KVM
cannot handle these traps and DTE access emulation fails.

Hence extend cmpxchg8b to perform cmpxchg16b when the destination memory
is 128 bit.

[1] https://github.com/AMDESE/qemu-iommu/blob/wip/for_iommufd_hw_queue-v8_amd_viommu_20260106/hw/i386/amd_viommu.c#L517

Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
---
 arch/x86/include/asm/uaccess.h |  2 +-
 arch/x86/kvm/emulate.c         | 50 +++++++++++++++++++++++-----------
 arch/x86/kvm/kvm_emulate.h     |  6 ++++
 arch/x86/kvm/x86.c             |  7 ++++-
 4 files changed, 47 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 6e10a8a3ba4d..948937a24cc5 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -598,7 +598,7 @@ do {										\
 
 extern void __try_cmpxchg_user_wrong_size(void);
 
-#if defined(CONFIG_X86_32) || !defined(X86_FEATURE_CX16)
+#ifdef CONFIG_X86_32
 /*
  * Always fail on 32 bit arch or 64 arch without 128 bit cmpxchg support
  */
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index c1b21282187f..535842336d76 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2184,24 +2184,36 @@ static int em_call_near_abs(struct x86_emulate_ctxt *ctxt)
 	return rc;
 }
 
+#define em_cmpxchg8b_16b(__c, rbits, mbits)\
+do {												\
+	u##mbits old = __c->dst.orig_val##mbits;						\
+												\
+	BUILD_BUG_ON(rbits * 2 != mbits);							\
+												\
+	if (((u##rbits) (old >> 0) != (u##rbits) reg_read(__c, VCPU_REGS_RAX)) ||		\
+	    ((u##rbits) (old >> rbits) != (u##rbits) reg_read(__c, VCPU_REGS_RDX))) {		\
+		*reg_write(__c, VCPU_REGS_RAX) = (u##rbits) (old >> 0);			\
+		*reg_write(__c, VCPU_REGS_RDX) = (u##rbits) (old >> rbits);			\
+		__c->eflags &= ~X86_EFLAGS_ZF;							\
+	} else {										\
+		__c->dst.val##mbits = ((u##mbits)reg_read(__c, VCPU_REGS_RCX) << rbits) |	\
+					(u##rbits) reg_read(__c, VCPU_REGS_RBX);		\
+												\
+		__c->eflags |= X86_EFLAGS_ZF;							\
+	}											\
+} while (0)
+
 static int em_cmpxchg8b(struct x86_emulate_ctxt *ctxt)
 {
-	u64 old = ctxt->dst.orig_val64;
-
-	if (ctxt->dst.bytes == 16)
+	if (WARN_ON_ONCE(8 + !!(ctxt->rex_bits & REX_W) * 8 != ctxt->dst.bytes))
 		return X86EMUL_UNHANDLEABLE;
 
-	if (((u32) (old >> 0) != (u32) reg_read(ctxt, VCPU_REGS_RAX)) ||
-	    ((u32) (old >> 32) != (u32) reg_read(ctxt, VCPU_REGS_RDX))) {
-		*reg_write(ctxt, VCPU_REGS_RAX) = (u32) (old >> 0);
-		*reg_write(ctxt, VCPU_REGS_RDX) = (u32) (old >> 32);
-		ctxt->eflags &= ~X86_EFLAGS_ZF;
-	} else {
-		ctxt->dst.val64 = ((u64)reg_read(ctxt, VCPU_REGS_RCX) << 32) |
-			(u32) reg_read(ctxt, VCPU_REGS_RBX);
-
-		ctxt->eflags |= X86_EFLAGS_ZF;
-	}
+	if (!(ctxt->rex_bits & REX_W))
+		em_cmpxchg8b_16b(ctxt, 32, 64);
+#ifdef CONFIG_X86_64
+	else
+		em_cmpxchg8b_16b(ctxt, 64, 128);
+#endif
 	return X86EMUL_CONTINUE;
 }
 
@@ -5414,8 +5426,14 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt, bool check_intercepts)
 			goto done;
 		}
 	}
-	/* Copy full 64-bit value for CMPXCHG8B.  */
-	ctxt->dst.orig_val64 = ctxt->dst.val64;
+	/* Copy full 64/128-bit value for CMPXCHG8B.  */
+
+#ifdef CONFIG_X86_64
+	if (ctxt->dst.bytes == 16)
+		ctxt->dst.orig_val128 = ctxt->dst.val128;
+	else
+#endif
+		ctxt->dst.orig_val64 = ctxt->dst.val64;
 
 special_insn:
 
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 3e375af15c03..89911845233d 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -263,6 +263,9 @@ struct operand {
 	union {
 		unsigned long orig_val;
 		u64 orig_val64;
+#ifdef CONFIG_X86_64
+		u128 orig_val128;
+#endif
 	};
 	union {
 		unsigned long *reg;
@@ -276,6 +279,9 @@ struct operand {
 	union {
 		unsigned long val;
 		u64 val64;
+#ifdef CONFIG_X86_64
+		u128 val128;
+#endif
 		char valptr[sizeof(avx256_t)];
 		sse128_t vec_val;
 		avx256_t vec_val2;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 79468ddfe473..60be16f05d79 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5271,7 +5271,7 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 	int r;
 
 	/* guests cmpxchg8b have to be emulated atomically */
-	if (bytes > 8 || (bytes & (bytes - 1)))
+	if (bytes > 2 * sizeof(unsigned long) || (bytes & (bytes - 1)))
 		goto emul_write;
 
 	gpa = kvm_mmu_gva_to_gpa_write(vcpu, addr, NULL);
@@ -5311,6 +5311,11 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 	case 8:
 		r = emulator_try_cmpxchg_user(u64, hva, old, new);
 		break;
+#ifdef CONFIG_X86_64
+	case 16:
+		r = emulator_try_cmpxchg_user(u128, hva, old, new);
+		break;
+#endif
 	default:
 		BUG();
 	}
-- 
2.34.1

Re: [PATCH v4 2/2] KVM: x86: Add support for cmpxchg16b emulation

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Missing guest CPUID check for X86_FEATURE_CX16 allows guest to invoke CMPXCHG16B emulation regardless of guest capabilities.
--

commit 17ba312dad40f9de38e63207a6f7c861db26c57a
Author: Sairaj Kodilkar <sarunkod@amd.com>
Subject: KVM: x86: Add support for cmpxchg16b emulation

This commit extends cmpxchg8b emulation in KVM to support the cmpxchg16b
instruction when the destination memory is 128-bit and REX.W is set. This
addresses an emulation failure where guests, such as the AMD IOMMU driver,
use 128-bit cmpxchg operations on MMIO regions.

Link: https://github.com/AMDESE/qemu-iommu/blob/wip/for_iommufd_hw_queue-v8_amd_viommu_20260106/hw/i386/amd_viommu.c#L517

> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index c1b21282187f3..535842336d766 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c

[ ... ]

> @@ -2184,24 +2184,36 @@ static int em_call_near_abs(struct x86_emulate_ctxt *ctxt)

[ ... ]

>  static int em_cmpxchg8b(struct x86_emulate_ctxt *ctxt)
>  {

[ ... ]

> -	if (ctxt->dst.bytes == 16)
> +	if (WARN_ON_ONCE(8 + !!(ctxt->rex_bits & REX_W) * 8 != ctxt->dst.bytes))
>  		return X86EMUL_UNHANDLEABLE;
>  
> -	if (((u32) (old >> 0) != (u32) reg_read(ctxt, VCPU_REGS_RAX)) ||
> -	    ((u32) (old >> 32) != (u32) reg_read(ctxt, VCPU_REGS_RDX))) {
> -		*reg_write(ctxt, VCPU_REGS_RAX) = (u32) (old >> 0);
> -		*reg_write(ctxt, VCPU_REGS_RDX) = (u32) (old >> 32);
> -		ctxt->eflags &= ~X86_EFLAGS_ZF;
> -	} else {
> -		ctxt->dst.val64 = ((u64)reg_read(ctxt, VCPU_REGS_RCX) << 32) |
> -			(u32) reg_read(ctxt, VCPU_REGS_RBX);
> -
> -		ctxt->eflags |= X86_EFLAGS_ZF;
> -	}
> +	if (!(ctxt->rex_bits & REX_W))
> +		em_cmpxchg8b_16b(ctxt, 32, 64);
> +#ifdef CONFIG_X86_64
> +	else
> +		em_cmpxchg8b_16b(ctxt, 64, 128);

[Severity: Medium]
Does this unconditionally allow a guest to invoke cmpxchg16b emulation
without checking if the guest CPUID actually supports X86_FEATURE_CX16?

If a hypervisor intentionally hides the CX16 CPUID feature to mimic older
hardware, it seems a guest could still execute a 128-bit cmpxchg targeting
an MMIO address, trapping to KVM and triggering this unconditional emulation
path in em_cmpxchg8b().

This appears to violate KVM guidelines requiring guest-visible features to be
discoverable and adhere to the guest CPUID configuration, which can break
live migration capability negotiation. Could we add a capability check for
X86_FEATURE_CX16 before performing the 128-bit emulation?

> +#endif
>  	return X86EMUL_CONTINUE;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908074739.10748-1-sarunkod@amd.com?part=2