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Thu, 17 Jul 2025 12:39:18 -0700 (PDT) Message-ID: <03ecaa72-2370-47f7-b84b-83831d188f4e@citrix.com> Date: Thu, 17 Jul 2025 20:39:17 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/6] x86/match-cpu: Support matching on steppings To: Jan Beulich Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Xen-devel References: <20250716173132.2213891-1-andrew.cooper3@citrix.com> <20250716173132.2213891-6-andrew.cooper3@citrix.com> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 17/07/2025 9:11 am, Jan Beulich wrote: > On 16.07.2025 19:31, Andrew Cooper wrote: >> --- a/xen/arch/x86/cpu/common.c >> +++ b/xen/arch/x86/cpu/common.c >> @@ -1003,13 +1003,15 @@ const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id table[]) >> const struct x86_cpu_id *m; >> const struct cpuinfo_x86 *c = &boot_cpu_data; >> >> - for (m = table; m->vendor | m->family | m->model | m->feature; m++) { >> + for (m = table; m->vendor | m->family | m->model | m->steppings | m->feature; m++) { > Nit: Line length. But - do we need the change at all? It looks entirely > implausible to me to use ->steppings with all of vendor, family, and > model being *_ANY (if, as per below, they would be 0 in the first place). I do keep on saying that | like this is pure obfuscation.  This is an excellent example. It's looking for the {} entry, by looking for 0's in all of the metadata fields.  A better check would be *(uint64_t *)m, or perhaps a unioned metadata field, but.. This is also a good demonstration of binary | is a bad thing to use, not only for legibility.  Swapping | for || lets the compiler do: add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-76 (-76) Function                                     old     new   delta x86_match_cpu                                243     167     -76 and the code generation looks much better too: https://termbin.com/c4m9 Although I'm a little confused as to why it's still done a split cmpw $0x0,(%rax) and cmpq $0xffff,(%rax) for the loop entry condition, when cmpq $0 would be the right one. > > Tangential: The ->feature check is slightly odd here. With everything > else being a wildcard (assuming these are 0; I can't find any X86_*_ANY > in the code base; INTEL_FAM6_ANY expands to X86_MODEL_ANY, but is itself > also not used anywhere), one wouldn't be able to use FPU, as that's > feature index 0. I notice though that ... > >> if (c->x86_vendor != m->vendor) >> continue; >> if (c->x86 != m->family) >> continue; >> if (c->x86_model != m->model) >> continue; > ... X86_*_ANY also aren't catered for here. Hence it remains unclear > what value those constants would actually be meant to have. > > Further tangential: The vendor check could in principle permit for > multiple vendors (e.g. AMD any Hygon at the same time), considering that > we use bit masks now. That would require the != there to change, though. In Linux, x86_cpu_id is a module ABI and has wildcards on all fields, because "please load me on any AMD Fam10 CPU" is something they want to express. In Xen, we only use it model/stepping specific lookup tables, so we don't need wildcards for V/F/M like Linux does. We do have a different layout of X86_VENDOR to Intel, and while that would allow us to merge an AMD and a Hygon row, I don't think anything good could come of trying. One problem Linux has is that X86_VENDOR_INTEL is 0, so they introduced a flags field with a VALID bit that now replaces the line of |'s.  I do not see any need for that in Xen. > >> --- a/xen/arch/x86/include/asm/match-cpu.h >> +++ b/xen/arch/x86/include/asm/match-cpu.h >> @@ -8,28 +8,32 @@ >> #include >> #include >> >> +#define X86_STEPPINGS_ANY 0 > Given the (deliberate aiui) plural, maybe better X86_STEPPINGS_ALL? Hmm, yeah, that's not great grammar.  I think I prefer X86_STEPPING_ANY to X86_STEPPINGS_ALL. > Also perhaps use 0xffff as the value, allowing to drop part of the > conditional in x86_match_cpu()? Interestingly, while it simplifies the C, it undoes most of the code generation improvements from switching | to ||. https://termbin.com/h0iu By removing the "m->steppings &&", gcc has now hoisted the load of c->stepping out of the loop (in fact, the whole 1U << c->stepping calculation), but that's now resulted in a spill/restore of %rbx in the loop, and also doubled up most of the loop.  I have no idea what it's trying to do here... > >> #define X86_FEATURE_ANY X86_FEATURE_LM >> >> struct x86_cpu_id { >> - uint16_t vendor; >> - uint16_t family; >> + uint8_t vendor; > Is shrinking this to 8 bits a good idea? We use 5 of them already. (Of > course we can re-enlarge later, if and when the need arises.) It's the same size as cpuinfo_x86's field has been for 2 decades. > >> + uint8_t family; > The family formula allows the value to be up to 0x10e. The return type > of get_cpu_family() is therefore wrong too, strictly speaking. As is > struct cpuinfo_x86's x86 field. Again, this is the size of the field in cpuinfo_x86.  I don't think 0x10e is anything we're going to have to worry about any time soon. > >> uint16_t model; > Whereas the model is strictly limited to 8 bits. There is space in here, if we need it, but you can't shrink it without breaking the check for the NULL entry (going back to the first obfuscation). ~Andrew