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Peter Anvin" , Borislav Petkov , Dave Hansen , Ingo Molnar , Thomas Gleixner , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org, suravee.suthikulpanit@amd.com, vasant.hegde@amd.com, nikunj.dadhania@amd.com, Manali.Shukla@amd.com References: <20260306102047.29760-1-sarunkod@amd.com> <99507d9b-c3be-4a77-8b84-79a30c7dd402@amd.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: <99507d9b-c3be-4a77-8b84-79a30c7dd402@amd.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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