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Thu, 17 Jul 2025 01:11:23 -0700 (PDT) Message-ID: Date: Thu, 17 Jul 2025 10:11:16 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/6] x86/match-cpu: Support matching on steppings To: Andrew Cooper 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-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <20250716173132.2213891-6-andrew.cooper3@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 16.07.2025 19:31, Andrew Cooper wrote: > Architecturally, stepping is a 4-bit field, so a uint16_t suffices for a > bitmap of steppings. > > In order to keep the size of struct x86_cpu_id the same, shrink the vendor and > family fields, neither of which need to be uint16_t in Xen. > > No functional change. > > Signed-off-by: Andrew Cooper > --- > CC: Jan Beulich > CC: Roger Pau Monné > > Linux supports all fields being optional. This has lead to using > X86_MATCH_CPU(ANY, ANY, ANY, ANY, FEATURE_FOO, NULL) in place of > boot_cpu_has(), and is not a construct I think we want to encorage. +1 > --- 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). 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. > --- 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? Also perhaps use 0xffff as the value, allowing to drop part of the conditional in x86_match_cpu()? > #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.) > + 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. > uint16_t model; Whereas the model is strictly limited to 8 bits. Jan