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

[Claudio Fontana]

Hi Paolo,

On 24.06.2016 14:53, Paolo Bonzini wrote:
> 
> 
> 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
> 

In my case the boot times are satisfactory including the PAM configuration loop 
in qboot.

The reason I removed the PAM backend in QEMU (or rather, made them configurable 
via existing CONFIG_PAM),
is as part of memory saving patches, not because of boottime issues.

Ciao,

Claudio