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[92.26.98.202]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-390e47965e8sm3445879f8f.1.2025.02.27.16.36.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Feb 2025 16:36:41 -0800 (PST) Message-ID: Date: Fri, 28 Feb 2025 00:36:40 +0000 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] x86/speculation: Simplify and make CALL_NOSPEC consistent To: Pawan Gupta Cc: x86@kernel.org, Josh Poimboeuf , linux-kernel@vger.kernel.org References: <20250226-call-nospec-v1-1-4dde04a5c7a7@linux.intel.com> <497a3694-cb0d-4678-9622-d9443bf8a40d@citrix.com> <20250227184133.lxm43awa2jgdpl4q@desk> <15253834-fb89-408f-8269-65413ad29f7a@citrix.com> <20250227231342.jh67quujcd3tgmft@desk> <20250228003117.q6f2f5a7ndt2o226@desk> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <20250228003117.q6f2f5a7ndt2o226@desk> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 28/02/2025 12:31 am, Pawan Gupta wrote: > On Thu, Feb 27, 2025 at 03:13:48PM -0800, Pawan Gupta wrote: >> On Thu, Feb 27, 2025 at 06:57:37PM +0000, Andrew Cooper wrote: >>> On 27/02/2025 6:41 pm, Pawan Gupta wrote: >>>> On Thu, Feb 27, 2025 at 12:49:48AM +0000, Andrew Cooper wrote: >>>>> On 26/02/2025 9:03 pm, Pawan Gupta wrote: >>>>>> @@ -420,20 +420,28 @@ static inline void call_depth_return_thunk(void) {} >>>>>> >>>>>> #ifdef CONFIG_X86_64 >>>>>> >>>>>> +/* >>>>>> + * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call >>>>>> + * to the retpoline thunk with a CS prefix when the register requires >>>>>> + * a REX prefix byte to encode. Also see apply_retpolines(). >>>>>> + */ >>>>> Technically, both comments aren't quite accurate.  __CS_PREFIX() emits a >>>>> conditional CS prefix in a manner compatible with >>>>> -mindirect-branch-cs-prefix, not the full 5/6 byte jmp/call. >>>> You are right, I will update the comment, and also the ASSEMBLY version >>>> where this comment came from: >>>> >>>> /* >>>> * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call >>>> * to the retpoline thunk with a CS prefix when the register requires >>>> * a REX prefix byte to encode. Also see apply_retpolines(). >>>> */ >>>> .macro __CS_PREFIX reg:req >>>> .irp rs,r8,r9,r10,r11,r12,r13,r14,r15 >>>> .ifc \reg,\rs >>>> .byte 0x2e >>>> .endif >>>> .endr >>>> .endm >>>> >>>>>> +#define __CS_PREFIX(reg) \ >>>>>> + .irp rs,r8,r9,r10,r11,r12,r13,r14,r15; \ >>>>>> + .ifc \\rs, \reg; \ >>>>> Why are these escaped differently?  Given they're all \r of some form or >>>>> another, I guess something is going wonky with __stringify(), but its >>>>> still weird for them to be different. >>>>> >>>>> Do you have a fully pre-processed source to hand to see how CALL_NOSPEC >>>>> ends up? >>>> Below is the pre-processed source for test_cc() generated with "make arch/x86/kvm/emulate.i". >>>> >>>> - This is with double backslash in ".ifc \\rs, \reg": >>>> >>>> asm("push %[flags]; popf; " ".irp rs,r8,r9,r10,r11,r12,r13,r14,r15; .ifc \\rs, \%V[thunk_target]; .byte 0x2e; .endif; .endr;" "call __x86_indirect_thunk_%V[thunk_target]\n" >>>> ^ >>>> This ends up emitting the CS prefix byte correctly: >>>> >>>> 2e e8 51 c9 32 01 cs call ffffffff824289e0 >>>> >>>> - This is with single backslash in ".ifc \\rs, \reg": >>>> >>>> asm("push %[flags]; popf; " ".irp rs,r8,r9,r10,r11,r12,r13,r14,r15; .ifc \rs, \%V[thunk_target]; .byte 0x2e; .endif; .endr;" "c all __x86_indirect_thunk_%V[thunk_target]\n" >>>> ^ >>>> This version does not emit the CS prefix byte: >>>> >>>> e8 52 c9 32 01 call ffffffff824289e0 >>>> >>>> I tried looking in gcc inline assembly documentation but could not find >>>> anything that would explain this. :( >>> It's because it's about plain C strings. >>> >>> \r (from \rs) is Carriage Return (ASCII 0x0d). >> Ah, right. >> >>> After AS's macro expansion, \reg becomes \% which is not a valid escape >>> character, so the \ gets left intact. >>> >>> \reg should become \\reg or you'll probably get a compiler complaining >>> eventually. >> Using \\ for reg like this: >> >> .ifc \\rs, \\reg >> >> is not emitting the CS prefix. I am trying to wrap my head around the >> magic. > Getting rid of one layer of macro makes it less magical: > > #define __CS_PREFIX(reg) \ > ".irp rs,r8,r9,r10,r11,r12,r13,r14,r15\n" \ > ".ifc \\rs," reg "\n" \ > ".byte 0x2e\n" \ > ".endif\n" \ > ".endr\n" > > #define CALL_NOSPEC __CS_PREFIX("%V[thunk_target]") \ > "call __x86_indirect_thunk_%V[thunk_target]\n" > #else > #define CALL_NOSPEC "call *%[thunk_target]\n" > #endif > > Preprocessor output is as expected, and this emits the CS prefix correctly: > > ".ifc \\rs, %V[thunk_target]" > > Full version: > > asm("push %[flags]; popf; " ".irp rs,r8,r9,r10,r11,r12,r13,r14,r15\n" ".ifc \\rs," "%V[thunk_target]" "\n" ".byte 0x2e\n" ".endif\n" ".endr\n" "call __x86_indirect_thunk_%V[thunk_target]\n" > : "=a"(rc), "+r" (current_stack_pointer) : [thunk_target]"r"(fop), [flags]"r"(flags)); Yeah, I think that's a lot better. I still cant spot precisely what's wrong with the prior \\reg. ~Andrew