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

Paolo Bonzini
2026-03-31T10:20:56Z

On 3/31/26 11:28, Sairaj Kodilkar wrote:
>
>
> On 3/12/2026 5:16 PM, Paolo Bonzini wrote:
>> On 3/6/26 16:38, Sean Christopherson wrote:
>>>> -    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;
>>
>>
>>> 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.
>>
>> It's actually pretty easy because a kind soul already wrote the
>> cmpxchg8b version of the macros.  Compile-tested only (and checked
>> that the asm does have the instruction):
>>
>> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/
>> uaccess.h
>> index 3a7755c1a4410..f40e815263233 100644
>> --- a/arch/x86/include/asm/uaccess.h
>> +++ b/arch/x86/include/asm/uaccess.h
>> @@ -388,14 +388,13 @@ do {                                    \
>>          *_old = __old;                        \
>>      likely(success);                    })
>>
>> -#ifdef CONFIG_X86_32
>> -#define __try_cmpxchg64_user_asm(_ptr, _pold, _new, label) ({    \
>> +#define __try_cmpxchg8b_16b_user_asm(_b, _ptr, _pold, _new, label)
>> ({    \
>>      bool success;                            \
>>      __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);        \
>>      __typeof__(*(_ptr)) __old = *_old;                \
>>      __typeof__(*(_ptr)) __new = (_new);                \
>>      asm_goto_output("\n"                        \
>> -             "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n"        \
>> +             "1: " LOCK_PREFIX "cmpxchg" #_b "b %[ptr]\n"    \
>
> Hi Paolo,
>
> I think its better to keep two implementations separate because this
> adds an additional complexity while passing the upper and lower half
> of the "_new"
> The cmpxchg8b expects ecx as "(u32)((u64)_new >> 32)" where as
> cmpxchg16b expects ecx as "(u64)((u128)_new >> 64)"

Yes, makes sense. Either way, you have the existing
__try_cmpxchg64_user_asm as a blueprint and that makes it much easier to
implement __try_cmpxchg128_user_asm.

Paolo