Re: [PATCH] KVM: x86: Add support for cmpxchg16b emulation
From: Sairaj Kodilkar
Date: Wed Mar 11 2026 - 07:19:29 EST
On 3/6/2026 9:08 PM, Sean Christopherson wrote:
On Fri, Mar 06, 2026, Sairaj Kodilkar wrote:
AMD and Intel both provides support for 128 bit cmpxchg operands usingPlease put this paragraph in the changelog proper, the "why" matters greatly here.
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@xxxxxxx>
---
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.
Sure.
QEMU implementation that traps DTE accesses:Uh, yeah, and the caller specifically pivoted on this size. I'm a-ok with a
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)
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;
Sure I'll update it.
return X86EMUL_UNHANDLEABLE;There is zero chance you properly tested this patch. Please write comprehensive
- 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;
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.
I think its not a too much of the work, I have a prototype ready which
implements cmpxchg16b access in uaccess.h. Tested with a KUT as well
seems to be working fine. Will post it once I cleanup the series.
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;
Yeah thanks, I completely missed that cmpxchg with lock prefix
consumes original value. I'll update the code.
+IMO we should use a macro to handle 8b vs. 16b. The code is going to be heinous
+ /* 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;
no matter what, and so to me, duplicating everything makes it twice as ugly,
whereas a macro makes it like 10% more ugly.
+ }Eh, I vote to keep it em_cmpxchg8b. _If_ we want to capture that its size is
+ 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,
variable, maybe em_cmpxchg8b_16b? em_cmpxchgxb just looks like a typo.
Yeah ok, I was not sure how to represent variable size hence used the x.
Will rename it.
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
--
Thanks
Sairaj