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Tue, 11 Nov 2025 09:55:57 -0800 (PST) Received: from [192.168.10.81] ([176.206.111.214]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-47782ceb0f9sm42703475e9.4.2025.11.11.09.55.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 11 Nov 2025 09:55:56 -0800 (PST) Message-ID: <6a093929-5e35-485a-934c-e0913d14ac14@redhat.com> Date: Tue, 11 Nov 2025 18:55:54 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC v1 16/20] KVM: x86: Decode REX2 prefix in the emulator To: "Chang S. Bae" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Cc: seanjc@google.com, chao.gao@intel.com, zhao1.liu@intel.com References: <20251110180131.28264-1-chang.seok.bae@intel.com> <20251110180131.28264-17-chang.seok.bae@intel.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: <20251110180131.28264-17-chang.seok.bae@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/10/25 19:01, Chang S. Bae wrote: > Extend the instruction emulator to recognize and interpret the REX2 > prefix byte. Also, detect and flag invalid prefix sequences after a REX2 > prefix. > > In the existing prefix-decoding loop, > * The loop exits when the first non-prefix byte is encountered. > * Any non-REX prefix clears previously recorded REX information. > > For REX2, however, once a REX2 prefix is encountered, most subsequent > prefixes are invalid. So, each subsequent prefix needs to be validated > before continuing the loop. > > Signed-off-by: Chang S. Bae > --- > RFC note: > The REX2 decoding itself is straightforward. The additional logic is > mainly to detect and handle invalid prefix sequences. If this seems > excessive, there is a chance to cut off this check since VMX would raise > '#UD' on such cases anyway. > --- > arch/x86/kvm/emulate.c | 24 +++++++++++++++++++++++- > 1 file changed, 23 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index 9bd61ea496e5..f9381a4055d6 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -4844,7 +4844,7 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int > ctxt->op_bytes = def_op_bytes; > ctxt->ad_bytes = def_ad_bytes; > > - /* Legacy prefixes. */ > + /* Legacy and REX/REX2 prefixes. */ > for (;;) { > switch (ctxt->b = insn_fetch(u8, ctxt)) { > case 0x66: /* operand-size override */ > @@ -4887,9 +4887,20 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int > case 0x40 ... 0x4f: /* REX */ > if (mode != X86EMUL_MODE_PROT64) > goto done_prefixes; > + if (ctxt->rex_prefix == REX2_PREFIX) > + break; > ctxt->rex_prefix = REX_PREFIX; > ctxt->rex.raw = 0x0f & ctxt->b; > continue; > + case 0xd5: /* REX2 */ > + if (mode != X86EMUL_MODE_PROT64) > + goto done_prefixes; Here you should also check if (ctxt->rex_prefix == REX_PREFIX) { ctxt->rex_prefix = REX2_INVALID; goto done_prefixes; } > + if (ctxt->rex_prefix == REX2_PREFIX && > + ctxt->rex.bits.m0 == 0) > + break; > + ctxt->rex_prefix = REX2_PREFIX; > + ctxt->rex.raw = insn_fetch(u8, ctxt); > + continue; After REX2 always comes the main opcode byte, so you can "goto done_prefixes" here. Or even jump here already; in pseudocode: ctxt->b = insn_fetch(u8, ctxt); if (rex2 & REX_M) goto decode_twobyte; else goto decode_onebyte; ... if (ctxt->b == 0x0f) { decode_twobyte: ... if (ctxt->b == 0x38 && ctxt->rex_prefix != REX2_PREFIX) ... } else { decode_onebyte: ... } > case 0xf0: /* LOCK */ > ctxt->lock_prefix = 1; > break; > @@ -4901,6 +4912,17 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int > goto done_prefixes; > } > > + if (ctxt->rex_prefix == REX2_PREFIX) { > + /* > + * A legacy or REX prefix following a REX2 prefix > + * forms an invalid byte sequences. Likewise, > + * a second REX2 prefix following a REX2 prefix > + * with M0=0 is invalid. > + */ > + ctxt->rex_prefix = REX2_INVALID; > + goto done_prefixes; > + } ... and this is not needed. Paolo > /* Any legacy prefix after a REX prefix nullifies its effect. */ > ctxt->rex_prefix = REX_NONE; > ctxt->rex.raw = 0;