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Date: Tue, 14 Sep 2021 10:04:47 +0800 From: Lai Jiangshan <laijs@...ux.alibaba.com> To: Andy Lutomirski <luto@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, the arch/x86 maintainers <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Dave Hansen <dave.hansen@...ux.intel.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, Dave Jiang <dave.jiang@...el.com>, Ben Widawsky <ben.widawsky@...el.com>, "Williams, Dan J" <dan.j.williams@...el.com>, Arvind Sankar <nivedita@...m.mit.edu> Subject: Re: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code On 2021/9/14 04:01, Andy Lutomirski wrote: > > > On Thu, Sep 2, 2021, at 3:50 AM, Lai Jiangshan wrote: >> From: Lai Jiangshan <laijs@...ux.alibaba.com> >> >> There is no constrain/limition to force native_load_gs_index() to be in >> ASM code. >> >> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com> > >> >> #ifdef CONFIG_X86_64 >> + >> +/* >> + * Reload gs selector with exception handling >> + * selector: new selector >> + * >> + * Is noinstr as it shouldn't be instrumented. >> + */ >> +noinstr void native_load_gs_index(unsigned int selector) >> +{ >> + unsigned long flags; >> + >> + local_irq_save(flags); > > This patch would be a bit less alarming if you moved the swapgs into asm. Emmm, this patch is not so clean and persuadable in C. I think Peter is still working on reworking the patchset and may be including improving this patch. I'm Okay if this patch is dropped. > Also, this needs a comment explaining why skipping the swapgs back to kernel gs in the exception path is correct. > I think it is all known that the exception handler in ASM_EXTABLE is running in kernel context where kernel gs is active. It does need a comment explaining why the label asm_load_gs_index_gs_change is needed, how does it help the error_entry() restores back to the kernel gs. Since the C-version error_entry() has to check asm_load_gs_index_gs_change, I think other way to handle the fault of "mov %gs" is just doing the %gs fixup in the C-version error_entry(). (see patch 11). it would be more directly, simple, and self-documented. Thank you for reviewing. >> + native_swapgs(); >> + asm volatile( >> + ".global asm_load_gs_index_gs_change \n" >> + "asm_load_gs_index_gs_change: \n" >> + "1: movl %0, %%gs \n" >> + " swapgs \n" >> + "2: \n" >> + _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_clear_gs) >> + :: "r" (selector) : "memory"); >> + alternative("", "mfence", X86_BUG_SWAPGS_FENCE); >> + local_irq_restore(flags); >> +} >> +
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