Re: [Qemu-devel] [PATCH v2] mips_malta: support up to 2GiB RAM

[Aurelien Jarno <aurelien@aurel32.net>]

On Fri, Sep 06, 2013 at 01:57:44PM +0100, Paul Burton wrote:
> A Malta board can support up to 2GiB of RAM. Since the unmapped kseg0/1
> regions are only 512MiB large & the latter 256MiB of those are taken up
> by the IO region, access to RAM beyond 256MiB must be done through a
> mapped region. In the case of a Linux guest this means we need to use
> highmem.
> 
> The mainline Linux kernel does not support highmem for Malta at this
> time, however this can be tested using the linux-mti-3.8 kernel branch
> available from:
> 
>   git://git.linux-mips.org/pub/scm/linux-mti.git
> 
> You should be able to boot a Linux kernel built from the linux-mti-3.8
> branch, with CONFIG_HIGHMEM enabled, using 2GiB RAM by passing "-m 2G"
> to QEMU and appending the following kernel parameters:
> 
>   mem=256m@0x0 mem=256m@0x90000000 mem=1536m@0x20000000
> 
> Note that the upper half of the physical address space of a Malta
> mirrors the lower half (hence the 2GiB limit) except that the IO region
> (0x10000000-0x1fffffff in the lower half) is not mirrored in the upper
> half. That is, physical addresses 0x90000000-0x9fffffff access RAM
> rather than the IO region, resulting in a physical address space
> resembling the following:
> 
>   0x00000000 -> 0x0fffffff  RAM
>   0x10000000 -> 0x1fffffff  I/O
>   0x20000000 -> 0x7fffffff  RAM
>   0x80000000 -> 0x8fffffff  RAM (mirror of 0x00000000 -> 0x0fffffff)
>   0x90000000 -> 0x9fffffff  RAM
>   0xa0000000 -> 0xffffffff  RAM (mirror of 0x20000000 -> 0x7fffffff)
> 
> The second mem parameter provided to the kernel above accesses the
> second 256MiB of RAM through the upper half of the physical address
> space, making use of the aliasing described above in order to avoid
> the IO region and use the whole 2GiB RAM.
> 
> The memory setup may be seen as 'backwards' in this commit since the
> 'real' memory is mapped in the upper half of the physical address space
> and the lower half contains the aliases. On real hardware it would be
> typical to see the upper half of the physical address space as the alias
> since the bus addresses generated match the lower half of the physical
> address space. However since the memory accessible in the upper half of
> the physical address space is uninterrupted by the IO region it is
> easiest to map the RAM as a whole there, and functionally it makes no
> difference to the target code.
> 
> Due to the requirements of accessing the second 256MiB of RAM through
> a mapping to the upper half of the physical address space it is usual
> for the bootloader to indicate a maximum of 256MiB memory to a kernel.
> This allows kernels which do not support such access to boot on systems
> with more than 256MiB of RAM. It is also the behaviour assumed by Linux.
> QEMUs small generated bootloader is modified to provide this behaviour.
> 
> Signed-off-by: Paul Burton <paul.burton@imgtec.com>
> Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
> Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
> ---
> Changes in v2:
>   - Add a table describing the physical memory space to the commit
>     message, as suggested by Aurelien Jarno.
> ---
>  hw/mips/mips_malta.c | 36 ++++++++++++++++++++++++++++--------
>  1 file changed, 28 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/mips/mips_malta.c b/hw/mips/mips_malta.c
> index ae0921c..05c8771 100644
> --- a/hw/mips/mips_malta.c
> +++ b/hw/mips/mips_malta.c
> @@ -827,7 +827,8 @@ static int64_t load_kernel (void)
>      }
>  
>      prom_set(prom_buf, prom_index++, "memsize");
> -    prom_set(prom_buf, prom_index++, "%i", loaderparams.ram_size);
> +    prom_set(prom_buf, prom_index++, "%i",
> +             MIN(loaderparams.ram_size, 256 << 20));
>      prom_set(prom_buf, prom_index++, "modetty0");
>      prom_set(prom_buf, prom_index++, "38400n8r");
>      prom_set(prom_buf, prom_index++, NULL);
> @@ -884,7 +885,9 @@ void mips_malta_init(QEMUMachineInitArgs *args)
>      char *filename;
>      pflash_t *fl;
>      MemoryRegion *system_memory = get_system_memory();
> -    MemoryRegion *ram = g_new(MemoryRegion, 1);
> +    MemoryRegion *ram_high = g_new(MemoryRegion, 1);
> +    MemoryRegion *ram_low_preio = g_new(MemoryRegion, 1);
> +    MemoryRegion *ram_low_postio;
>      MemoryRegion *bios, *bios_copy = g_new(MemoryRegion, 1);
>      target_long bios_size = FLASH_SIZE;
>      const size_t smbus_eeprom_size = 8 * 256;
> @@ -951,15 +954,32 @@ void mips_malta_init(QEMUMachineInitArgs *args)
>      env = &cpu->env;
>  
>      /* allocate RAM */
> -    if (ram_size > (256 << 20)) {
> +    if (ram_size > (2048u << 20)) {
>          fprintf(stderr,
> -                "qemu: Too much memory for this machine: %d MB, maximum 256 MB\n",
> +                "qemu: Too much memory for this machine: %d MB, maximum 2048 MB\n",
>                  ((unsigned int)ram_size / (1 << 20)));
>          exit(1);
>      }
> -    memory_region_init_ram(ram, NULL, "mips_malta.ram", ram_size);
> -    vmstate_register_ram_global(ram);
> -    memory_region_add_subregion(system_memory, 0, ram);
> +
> +    /* register RAM at high address where it is undisturbed by IO */
> +    memory_region_init_ram(ram_high, NULL, "mips_malta.ram", ram_size);
> +    vmstate_register_ram_global(ram_high);
> +    memory_region_add_subregion(system_memory, 0x80000000, ram_high);
> +
> +    /* alias for pre IO hole access */
> +    memory_region_init_alias(ram_low_preio, NULL, "mips_malta_low_preio.ram",
> +                             ram_high, 0, MIN(ram_size, (256 << 20)));
> +    memory_region_add_subregion(system_memory, 0, ram_low_preio);
> +
> +    /* alias for post IO hole access, if there is enough RAM */
> +    if (ram_size > (512 << 20)) {
> +        ram_low_postio = g_new(MemoryRegion, 1);
> +        memory_region_init_alias(ram_low_postio, NULL,
> +                                 "mips_malta_low_postio.ram",
> +                                 ram_high, 512 << 20,
> +                                 ram_size - (512 << 20));
> +        memory_region_add_subregion(system_memory, 512 << 20, ram_low_postio);
> +    }
>  
>      /* generate SPD EEPROM data */
>      generate_eeprom_spd(&smbus_eeprom_buf[0 * 256], ram_size);
> @@ -992,7 +1012,7 @@ void mips_malta_init(QEMUMachineInitArgs *args)
>      fl_idx++;
>      if (kernel_filename) {
>          /* Write a small bootloader to the flash location. */
> -        loaderparams.ram_size = ram_size;
> +        loaderparams.ram_size = MIN(ram_size, 256 << 20);
>          loaderparams.kernel_filename = kernel_filename;
>          loaderparams.kernel_cmdline = kernel_cmdline;
>          loaderparams.initrd_filename = initrd_filename;

Thanks, applied.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net