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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> 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 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" \ _ASM_EXTABLE_UA(1b, %l[label]) \ : CC_OUT(z) (success), \ "+A" (__old), \ @@ -407,7 +406,6 @@ do { \ if (unlikely(!success)) \ *_old = __old; \ 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) ({ \ int __err = 0; \ @@ -433,7 +431,6 @@ do { \ *_old = __old; \ likely(success); }) -#ifdef CONFIG_X86_32 /* * Unlike the normal CMPXCHG, use output GPR for both success/fail and error. * There are only six GPRs available and four (EAX, EBX, ECX, and EDX) are @@ -441,13 +438,13 @@ do { \ * both ESI and EDI for the memory operand, compilation will fail if the error * is an input+output as there will be no register available for input. */ -#define __try_cmpxchg64_user_asm(_ptr, _pold, _new, label) ({ \ +#define __try_cmpxchg8b_16b_user_asm(_b, _ptr, _pold, _new, label) ({ \ int __result; \ __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \ __typeof__(*(_ptr)) __old = *_old; \ __typeof__(*(_ptr)) __new = (_new); \ asm volatile("\n" \ - "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n" \ + "1: " LOCK_PREFIX "cmpxchg" #_b "b %[ptr]\n" \ "mov $0, %[result]\n\t" \ "setz %b[result]\n" \ "2:\n" \ @@ -464,7 +461,6 @@ do { \ if (unlikely(!__result)) \ *_old = __old; \ likely(__result); }) -#endif // CONFIG_X86_32 #endif // CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT /* FIXME: this hack is definitely wrong -AK */ @@ -552,9 +549,15 @@ do { \ extern void __try_cmpxchg_user_wrong_size(void); -#ifndef CONFIG_X86_32 +#ifdef CONFIG_X86_32 +#define __try_cmpxchg64_user_asm(_ptr, _pold, _new, label) \ + __try_cmpxchg8b_16b_user_asm(8, _ptr, _pold, _new, label) +#else #define __try_cmpxchg64_user_asm(_ptr, _oldp, _nval, _label) \ __try_cmpxchg_user_asm("q", "r", (_ptr), (_oldp), (_nval), _label) + +#define __try_cmpxchg128_user_asm(_ptr, _pold, _new, label) \ + __try_cmpxchg8b_16b_user_asm(16, _ptr, _pold, _new, label) #endif /* @@ -581,6 +584,9 @@ extern void __try_cmpxchg_user_wrong_size(void); case 8: __ret = __try_cmpxchg64_user_asm((__force u64 *)(_ptr), (_oldp),\ (_nval), _label); \ break; \ + case 16:__ret = __try_cmpxchg128_user_asm((__force u128 *)(_ptr), (_oldp),\ + (_nval), _label); \ + break; \ default: __try_cmpxchg_user_wrong_size(); \ } \ __ret; }) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index f2db04aa6a17f..3e1bd9cdd75e6 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -7973,7 +7973,7 @@ 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); @@ -8013,6 +8013,11 @@ 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(); } (with the uaccess.h part to be split in a separate patch, of course). Paolo