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Thu, 14 Aug 2025 00:26:55 -0700 (PDT) Message-ID: Date: Thu, 14 Aug 2025 09:26:53 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 09/22] x86/traps: Move load_system_tables() into traps-setup.c To: Andrew Cooper Cc: Xen-devel , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Nicola Vetrini References: <20250808202314.1045968-1-andrew.cooper3@citrix.com> <20250808202314.1045968-10-andrew.cooper3@citrix.com> <80285a06f5d4fb3bd3e378317cca61ca@bugseng.com> <7e906964-b07d-4205-a683-e665f19d5edc@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: <7e906964-b07d-4205-a683-e665f19d5edc@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 13.08.2025 13:36, Andrew Cooper wrote: > On 12/08/2025 10:43 am, Nicola Vetrini wrote: >> On 2025-08-08 22:23, Andrew Cooper wrote: >>> diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c >>> index 8ca379c9e4cb..13b8fcf0ba51 100644 >>> --- a/xen/arch/x86/traps-setup.c >>> +++ b/xen/arch/x86/traps-setup.c >>> @@ -19,6 +20,124 @@ boolean_param("ler", opt_ler); >>> >>>  void nocall entry_PF(void); >>> >>> +/* >>> + * Sets up system tables and descriptors for IDT devliery. >>> + * >>> + * - Sets up TSS with stack pointers, including ISTs >>> + * - Inserts TSS selector into regular and compat GDTs >>> + * - Loads GDT, IDT, TR then null LDT >>> + * - Sets up IST references in the IDT >>> + */ >>> +static void load_system_tables(void) >>> +{ >>> +    unsigned int i, cpu = smp_processor_id(); >>> +    unsigned long stack_bottom = get_stack_bottom(), >>> +        stack_top = stack_bottom & ~(STACK_SIZE - 1); >>> +    /* >>> +     * NB: define tss_page as a local variable because clang 3.5 >>> doesn't >>> +     * support using ARRAY_SIZE against per-cpu variables. >>> +     */ >>> +    struct tss_page *tss_page = &this_cpu(tss_page); >>> +    idt_entry_t *idt = this_cpu(idt); >>> + >> >> Given the clang baseline this might not be needed anymore? > > Hmm.  While true, looking at 51461114e26, the code is definitely better > written with the tss_page variable and we wouldn't want to go back to > the old form. > > I think that I'll simply drop the comment. > > ~Andrew > > P.S. > > Generally speaking, because of the RELOC_HIDE() in this_cpu(), any time > you ever want two accesses to a variable, it's better (code gen wise) to > construct a pointer to it and use the point multiple times. > > I don't understand why there's a RELOC_HIDE() in this_cpu().  The > justification doesn't make sense, but I've not had time to explore what > happens if we take it out. There's no justification in xen/percpu.h? My understanding is that we simply may not expose any accesses to per_cpu_* variables directly to the compiler, or there's a risk that it might access the "master" variable (i.e. CPU0's on at least x86). Jan