rust-raspberrypi-OS-tutorials/.10_virtualmemory/mmu.c

/*
 * Copyright (C) 2018 bzt (bztsrc@github)
 *
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy,
 * modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 *
 */

#include "gpio.h"             // get MMIO_BASE
#include "uart.h"

#define PAGESIZE    4096

// granularity
#define PT_PAGE     0b11        // 4k granule
#define PT_BLOCK    0b01        // 2M granule
// accessibility
#define PT_KERNEL   (0<<6)      // privileged, supervisor EL1 access only
#define PT_USER     (1<<6)      // unprivileged, EL0 access allowed
#define PT_RW       (0<<7)      // read-write
#define PT_RO       (1<<7)      // read-only
#define PT_AF       (1<<10)     // accessed flag
#define PT_NX       (1UL<<54)   // no execute
// shareability
#define PT_OSH      (2<<8)      // outter shareable
#define PT_ISH      (3<<8)      // inner shareable
// defined in MAIR register
#define PT_MEM      (0<<2)      // normal memory
#define PT_DEV      (1<<2)      // device MMIO
#define PT_NC       (2<<2)      // non-cachable

#define TTBR_ENABLE 1

// get addresses from linker
extern volatile unsigned char _data;
extern volatile unsigned char _end;

/**
 * Set up page translation tables and enable virtual memory
 */
void mmu_init()
{
    unsigned long r, b, *paging=(unsigned long*)&_end;

    /* create MMU translation tables at _end */

    // TTBR0, identity L1
    paging[0]=(unsigned long)((unsigned char*)&_end+2*PAGESIZE) |    // physical address
        PT_PAGE |     // it has the "Present" flag, which must be set, and we have area in it mapped by pages
        PT_AF |       // accessed flag. Without this we're going to have a Data Abort exception
        PT_USER |     // non-privileged
        PT_ISH |      // inner shareable
        PT_MEM;       // normal memory

    // identity L2, first 2M block
    paging[2*512]=(unsigned long)((unsigned char*)&_end+3*PAGESIZE) | // physical address
        PT_PAGE |     // we have area in it mapped by pages
        PT_AF |       // accessed flag
        PT_USER |     // non-privileged
        PT_ISH |      // inner shareable
        PT_MEM;       // normal memory

    // identity L2 2M blocks
    b=MMIO_BASE>>21;
    // skip 0th, as we're about to map it by L3
    for(r=1;r<512;r++)
        paging[2*512+r]=(unsigned long)((r<<21)) |  // physical address
        PT_BLOCK |    // map 2M block
        PT_AF |       // accessed flag
        PT_NX |       // no execute
        PT_USER |     // non-privileged
        (r>=b? PT_OSH|PT_DEV : PT_ISH|PT_MEM); // different attributes for device memory

    // identity L3
    for(r=0;r<512;r++)
        paging[3*512+r]=(unsigned long)(r*PAGESIZE) |   // physical address
        PT_PAGE |     // map 4k
        PT_AF |       // accessed flag
        PT_USER |     // non-privileged
        PT_ISH |      // inner shareable
        ((r<0x80||r>(unsigned long)&_data/PAGESIZE)? PT_RW|PT_NX : PT_RO); // different for code and data

    // TTBR1, kernel L1
    paging[512+511]=(unsigned long)((unsigned char*)&_end+4*PAGESIZE) | // physical address
        PT_PAGE |     // we have area in it mapped by pages
        PT_AF |       // accessed flag
        PT_KERNEL |   // privileged
        PT_ISH |      // inner shareable
        PT_MEM;       // normal memory

    // kernel L2
    paging[4*512+511]=(unsigned long)((unsigned char*)&_end+5*PAGESIZE) |   // physical address
        PT_PAGE |     // we have area in it mapped by pages
        PT_AF |       // accessed flag
        PT_KERNEL |   // privileged
        PT_ISH |      // inner shareable
        PT_MEM;       // normal memory

    // kernel L3
    paging[5*512]=(unsigned long)(MMIO_BASE+0x00201000) |   // physical address
        PT_PAGE |     // map 4k
        PT_AF |       // accessed flag
        PT_NX |       // no execute
        PT_KERNEL |   // privileged
        PT_OSH |      // outter shareable
        PT_DEV;       // device memory

    /* okay, now we have to set system registers to enable MMU */

    // check for 4k granule and at least 36 bits physical address bus */
    asm volatile ("mrs %0, id_aa64mmfr0_el1" : "=r" (r));
    b=r&0xF;
    if(r&(0xF<<28)/*4k*/ || b<1/*36 bits*/) {
        uart_puts("ERROR: 4k granule or 36 bit address space not supported\n");
        return;
    }

    // first, set Memory Attributes array, indexed by PT_MEM, PT_DEV, PT_NC in our example
    r=  (0xFF << 0) |    // AttrIdx=0: normal, IWBWA, OWBWA, NTR
        (0x04 << 8) |    // AttrIdx=1: device, nGnRE (must be OSH too)
        (0x44 <<16);     // AttrIdx=2: non cacheable
    asm volatile ("msr mair_el1, %0" : : "r" (r));

    // next, specify mapping characteristics in translate control register
    r=  (0b00LL << 37) | // TBI=0, no tagging
        (b << 32) |      // IPS=autodetected
        (0b10LL << 30) | // TG1=4k
        (0b11LL << 28) | // SH1=3 inner
        (0b01LL << 26) | // ORGN1=1 write back
        (0b01LL << 24) | // IRGN1=1 write back
        (0b0LL  << 23) | // EPD1 enable higher half
        (25LL   << 16) | // T1SZ=25, 3 levels (512G)
        (0b00LL << 14) | // TG0=4k
        (0b11LL << 12) | // SH0=3 inner
        (0b01LL << 10) | // ORGN0=1 write back
        (0b01LL << 8) |  // IRGN0=1 write back
        (0b0LL  << 7) |  // EPD0 enable lower half
        (25LL   << 0);   // T0SZ=25, 3 levels (512G)
    asm volatile ("msr tcr_el1, %0; isb" : : "r" (r));

    // tell the MMU where our translation tables are. TTBR_ENABLE bit not documented, but required
    // lower half, user space
    asm volatile ("msr ttbr0_el1, %0" : : "r" ((unsigned long)&_end + TTBR_ENABLE));
    // upper half, kernel space
    asm volatile ("msr ttbr1_el1, %0" : : "r" ((unsigned long)&_end + TTBR_ENABLE + PAGESIZE));

    // finally, toggle some bits in system control register to enable page translation
    asm volatile ("dsb ish; isb; mrs %0, sctlr_el1" : "=r" (r));
    r|=0xC00800;     // set mandatory reserved bits
    r&=~((1<<25) |   // clear EE, little endian translation tables
         (1<<24) |   // clear E0E
         (1<<19) |   // clear WXN
         (1<<12) |   // clear I, no instruction cache
         (1<<4) |    // clear SA0
         (1<<3) |    // clear SA
         (1<<2) |    // clear C, no cache at all
         (1<<1));    // clear A, no aligment check
    r|=  (1<<0);     // set M, enable MMU
    asm volatile ("msr sctlr_el1, %0; isb" : : "r" (r));
}