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

Sean Christopherson
2026-03-06T15:38:28Z

On Fri, Mar 06, 2026, Sairaj Kodilkar wrote:
> 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.
>
> Hence extend cmpxchg8b to perform cmpxchg16b when the destination memory
> is 128 bit.
>
> Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
> ---
> Background:
>
> The AMD IOMMU driver writes 256-bit device table entries with two
> 128-bit cmpxchg operations. For guests using hardware-accelerated
> vIOMMU (still in progress), QEMU traps device table accesses to set up
> nested page tables. Without 128-bit cmpxchg emulation, KVM cannot
> handle these traps and DTE access emulation fails.

Please put this paragraph in the changelog proper, the "why" matters greatly here.

> QEMU implementation that traps DTE accesses:
> https://github.com/AMDESE/qemu-iommu/blob/wip/for_iommufd_hw_queue-v8_amd_viommu_20260106/hw/i386/amd_viommu.c#L517
> ---
> arch/x86/kvm/emulate.c | 48 +++++++++++++++++++++++++++++++-------
> arch/x86/kvm/kvm_emulate.h | 1 +
> 2 files changed, 41 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index c8e292e9a24d..e1a08cd3274b 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2188,17 +2188,18 @@ static int em_call_near_abs(struct x86_emulate_ctxt *ctxt)
> return rc;
> }
>
> -static int em_cmpxchg8b(struct x86_emulate_ctxt *ctxt)
> +static int __handle_cmpxchg8b(struct x86_emulate_ctxt *ctxt)
> {
> - u64 old = ctxt->dst.orig_val64;
> + u64 old64 = ctxt->dst.orig_val64;
>
> - if (ctxt->dst.bytes == 16)
> + /* Use of the REX.W prefix promotes operation to 128 bits */
> + if (ctxt->rex_bits & REX_W)

Uh, yeah, and the caller specifically pivoted on this size. I'm a-ok with a
sanity check, but it should be exactly that, e.g.

if (WARN_ON_ONCE(8 + (ctxt->rex_bits & REX_W) * 8 != ctxt->dst.bytes))
return X86EMUL_UNHANDLEABLE;

> 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);
> + if (((u32) (old64 >> 0) != (u32) reg_read(ctxt, VCPU_REGS_RAX)) ||
> + ((u32) (old64 >> 32) != (u32) reg_read(ctxt, VCPU_REGS_RDX))) {
> + *reg_write(ctxt, VCPU_REGS_RAX) = (u32) (old64 >> 0);
> + *reg_write(ctxt, VCPU_REGS_RDX) = (u32) (old64 >> 32);
> ctxt->eflags &= ~X86_EFLAGS_ZF;
> } else {
> ctxt->dst.val64 = ((u64)reg_read(ctxt, VCPU_REGS_RCX) << 32) |
> @@ -2209,6 +2210,37 @@ static int em_cmpxchg8b(struct x86_emulate_ctxt *ctxt)
> return X86EMUL_CONTINUE;
> }
>
> +static int __handle_cmpxchg16b(struct x86_emulate_ctxt *ctxt)
> +{
> + __uint128_t old128 = ctxt->dst.val128;

There is zero chance you properly tested this patch. Please write comprehensive
testcases in KUT's emulator64.c before posting the next version.

In writeback(), the OP_MEM case quite clearly operates on "unsigned long" values
*and* consumes orig_val in the locked case.

case OP_MEM:
if (ctxt->lock_prefix)
return segmented_cmpxchg(ctxt,
op->addr.mem,
&op->orig_val,
&op->val,
op->bytes);
else
return segmented_write(ctxt,
op->addr.mem,
&op->val,
op->bytes);

And then emulator_cmpxchg_emulated() should be taught to do cmpxchg16b itself:

/* guests cmpxchg8b have to be emulated atomically */
if (bytes > 8 || (bytes & (bytes - 1)))
goto emul_write;

That might be a big lift, e.g. to get a 16-byte uaccess version. If it's
unreasonably difficult, we should reject emulation of LOCK CMPXCHG16B.

And this code would also need to be updated to copy the full 128-bit value.

/* Copy full 64-bit value for CMPXCHG8B. */
ctxt->dst.orig_val64 = ctxt->dst.val64;

Maybe something like this? Or maybe just copy 128 bits unconditionally (I'm not
sure if that could read uninitiatlized data or not).

/* Copy the full 64/128-bit value for CMPXCHG8B/16B. */
if (IS_ENABLED(CONFIG_X86_64) && ctxt->dst.bytes == 16)
ctxt->dst.orig_val128 = ctxt->dst.val128;
else
ctxt->dst.orig_val64 = ctxt->dst.val64;

> +
> + /* Use of the REX.W prefix promotes operation to 128 bits */
> + if (!(ctxt->rex_bits & REX_W))
> + return X86EMUL_UNHANDLEABLE;
> +
> + if (((u64) (old128 >> 0) != (u64) reg_read(ctxt, VCPU_REGS_RAX)) ||
> + ((u64) (old128 >> 64) != (u64) reg_read(ctxt, VCPU_REGS_RDX))) {
> + *reg_write(ctxt, VCPU_REGS_RAX) = (u64) (old128 >> 0);
> + *reg_write(ctxt, VCPU_REGS_RDX) = (u64) (old128 >> 64);
> + ctxt->eflags &= ~X86_EFLAGS_ZF;
> + } else {
> + ctxt->dst.val128 =
> + ((__uint128_t) reg_read(ctxt, VCPU_REGS_RCX) << 64) |
> + (u64) reg_read(ctxt, VCPU_REGS_RBX);
> +
> + ctxt->eflags |= X86_EFLAGS_ZF;

IMO we should use a macro to handle 8b vs. 16b. The code is going to be heinous
no matter what, and so to me, duplicating everything makes it twice as ugly,
whereas a macro makes it like 10% more ugly.

> + }
> + return X86EMUL_CONTINUE;
> +}
> +
> +static int em_cmpxchgxb(struct x86_emulate_ctxt *ctxt)
> +{
> + if (ctxt->dst.bytes == 16)
> + return __handle_cmpxchg16b(ctxt);
> +
> + return __handle_cmpxchg8b(ctxt);
> +}
> +
> static int em_ret(struct x86_emulate_ctxt *ctxt)
> {
> int rc;
> @@ -4097,7 +4129,7 @@ static const struct gprefix pfx_0f_c7_7 = {
>
>
> static const struct group_dual group9 = { {
> - N, I(DstMem64 | Lock | PageTable, em_cmpxchg8b), N, N, N, N, N, N,
> + N, I(DstMem64 | Lock | PageTable, em_cmpxchgxb), N, N, N, N, N, N,

Eh, I vote to keep it em_cmpxchg8b. _If_ we want to capture that its size is
variable, maybe em_cmpxchg8b_16b? em_cmpxchgxb just looks like a typo.

Sans correct writeback() handling, something like so:

---
arch/x86/kvm/emulate.c | 44 ++++++++++++++++++++++++--------------
arch/x86/kvm/kvm_emulate.h | 2 ++
2 files changed, 30 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 6145dac4a605..3ec4555571fa 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2201,24 +2201,33 @@ 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);
+ else
+ em_cmpxchg8b_16b(ctxt, 64, 128);
return X86EMUL_CONTINUE;
}

@@ -5418,8 +5427,11 @@ 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 the full 64/128-bit value for CMPXCHG8B/16B. */
+ if (IS_ENABLED(CONFIG_X86_64) && ctxt->dst.bytes == 16)
+ ctxt->dst.orig_val128 = ctxt->dst.val128;
+ else
+ 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 fb3dab4b5a53..0e9968319343 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -255,6 +255,7 @@ struct operand {
union {
unsigned long orig_val;
u64 orig_val64;
+ u64 orig_val128;
};
union {
unsigned long *reg;
@@ -268,6 +269,7 @@ struct operand {
union {
unsigned long val;
u64 val64;
+ __uint128_t val128;
char valptr[sizeof(avx256_t)];
sse128_t vec_val;
avx256_t vec_val2;

base-commit: 5128b972fb2801ad9aca54d990a75611ab5283a9
--