Re: [PATCH v3 14/22] microvm: use 2G split unconditionally

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 20 May 2020 15:19:55 +0200
Gerd Hoffmann <kraxel@redhat.com> wrote:

> Looks like the logiv was copied over from q35.
> 
> q35 does this for backward compatibility, there is no reason to do this
> on microvm though.  So split @ 2G unconditionally.

not related to your ACPI rework, but just an idea for future of microvm

I wonder if we should carry over all this fixed RAM layout legacy from pc/q35
with a bunch of knobs to tweak it (along with complicated logic).

Can we just re-use pc-dimms for main RAM and let user specify RAM layout the way they wish?


> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  hw/i386/microvm.c | 16 +---------------
>  1 file changed, 1 insertion(+), 15 deletions(-)
> 
> diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> index 867d3d652145..b8f0d3283758 100644
> --- a/hw/i386/microvm.c
> +++ b/hw/i386/microvm.c
> @@ -170,23 +170,9 @@ static void microvm_memory_init(MicrovmMachineState *mms)
>      MemoryRegion *ram_below_4g, *ram_above_4g;
>      MemoryRegion *system_memory = get_system_memory();
>      FWCfgState *fw_cfg;
> -    ram_addr_t lowmem;
> +    ram_addr_t lowmem = 0x80000000; /* 2G */
>      int i;
>  
> -    /*
> -     * Check whether RAM fits below 4G (leaving 1/2 GByte for IO memory
> -     * and 256 Mbytes for PCI Express Enhanced Configuration Access Mapping
> -     * also known as MMCFG).
> -     * If it doesn't, we need to split it in chunks below and above 4G.
> -     * In any case, try to make sure that guest addresses aligned at
> -     * 1G boundaries get mapped to host addresses aligned at 1G boundaries.
> -     */
> -    if (machine->ram_size >= 0xb0000000) {
> -        lowmem = 0x80000000;
> -    } else {
> -        lowmem = 0xb0000000;
> -    }
> -
>      /*
>       * Handle the machine opt max-ram-below-4g.  It is basically doing
>       * min(qemu limit, user limit).