Re: [Qemu-devel] [RFC 0/9] Introduce light weight PC platform pc-lite

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/06/2016 08:41, Claudio Fontana wrote:
>> I use qboot for similar goals, you mention that PAM is necessary because of how qboot probes parallel flash,
>> however in my custom platform I removed PAM completely from QEMU, and everything seems to work without any problems..
> 
> Btw before you ask: yes I am booting with pflash.

By default low memory points to PCI address space

    00000000000f0000-00000000000fffff (prio 1, RW): alias pam-ram @pc.ram 00000000000f0000-00000000000fffff [disabled]
    00000000000f0000-00000000000fffff (prio 1, RW): alias pam-pci @pc.ram 00000000000f0000-00000000000fffff [disabled]
    00000000000f0000-00000000000fffff (prio 1, R-): alias pam-rom @pc.ram 00000000000f0000-00000000000fffff [disabled]
    00000000000f0000-00000000000fffff (prio 1, RW): alias pam-pci @pci 00000000000f0000-00000000000fffff

All that qboot does is enabling pam-ram:

        // Make ram from 0xc0000-0xf0000 read-write
        int i;
        for (i=0; i<6; i++) {
                int pam = pambase + 1 + i;
                pci_config_writeb(bdf, pam, 0x33);
        }

        // Make ram from 0xf0000-0x100000 read-write and shadow BIOS
        // We're still running from 0xffff0000
        pci_config_writeb(bdf, pambase, 0x30);
        memcpy(low_start, bios_start, 0x10000);

So if you remove PAM but you are leaving 0xC000-0x10000 pointing to
RAM, you are effectively moving qboot's PAM configuration to QEMU. :)

Of these writes, only the last write is strictly necessary.  qboot
currently uses 0xe0000-0xf0000 for the ACPI tables but we could move
them to the EBDA instead and save the initial loop.  But I'd like to
see a trace saying how much time is spent configuring PAM exactly.

Paolo