linux-kernel - Re: [PATCH v3 11/20] arm64: mm: Map entry trampoline into trampoline and kernel page tables

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Message-ID: <20180123100447.GB1686@arm.com>
Date:   Tue, 23 Jan 2018 10:04:47 +0000
From:   Will Deacon <will.deacon@....com>
To:     Yisheng Xie <xieyisheng1@...wei.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, mark.rutland@....com,
        ard.biesheuvel@...aro.org, sboyd@...eaurora.org,
        dave.hansen@...ux.intel.com, keescook@...omium.org,
        msalter@...hat.com, labbott@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH v3 11/20] arm64: mm: Map entry trampoline into trampoline
 and kernel page tables

On Tue, Jan 23, 2018 at 04:28:45PM +0800, Yisheng Xie wrote:
> On 2017/12/6 20:35, Will Deacon wrote:
> > +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > +static int __init map_entry_trampoline(void)
> > +{
> > +	extern char __entry_tramp_text_start[];
> > +
> > +	pgprot_t prot = rodata_enabled ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;
> > +	phys_addr_t pa_start = __pa_symbol(__entry_tramp_text_start);
> > +
> > +	/* The trampoline is always mapped and can therefore be global */
> > +	pgprot_val(prot) &= ~PTE_NG;
> > +
> > +	/* Map only the text into the trampoline page table */
> > +	memset(tramp_pg_dir, 0, PGD_SIZE);
> > +	__create_pgd_mapping(tramp_pg_dir, pa_start, TRAMP_VALIAS, PAGE_SIZE,
> > +			     prot, pgd_pgtable_alloc, 0);
> 
> How the tramp_pg_dir is used, should it be set to ttbr1 when exit kernel? Sorry
> for I do not find where it is used.

Yes, that's what happens when we return to userspace. The code is a little
convoluted, but the tramp_pg_dir is placed at a fixed offset from swapper
(see the linker script) so the sub instruction in tramp_unmap_kernel is what
gives us the ttbr1 value we need.

Will