Re: [Qemu-devel] [PATCH v2] map 64-bit PCI BARs at location provided by emulator

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Oct 13, 2013 at 05:11:54PM +0200, Igor Mammedov wrote:
> On Sun, 13 Oct 2013 15:31:23 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Sun, Oct 13, 2013 at 02:13:44PM +0200, Igor Mammedov wrote:
> > > Currently 64-bit PCI BARs are unconditionally mapped by BIOS right
> > > over 4G + RamSizeOver4G location, which doesn't allow to reserve
> > > extra space before 64-bit PCI window. For memory hotplug an extra
> > > RAM space might be reserved after present 64-bit RAM end and BIOS
> > > should map 64-bit PCI BARs after it.
> > > 
> > > Introduce "etc/pcimem64-start" romfile to provide BIOS a hint
> > > where it should start mapping of 64-bit PCI BARs. If romfile is
> > > missing, BIOS reverts to legacy behavior and starts mapping right
> > > after high memory.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > > v2:
> > >   * place 64-bit window behind high RAM end if "etc/pcimem64-start"
> > >     points below it.
> > 
> > Hmm I had an alternative suggestion of passing smbios
> > tables in from QEMU (seabios already has a mechanism for this).
> > We could then simply increase the existing RamSize variable.
> > 
> > What do you think about this idea?
> There is several issues with RamSizeOver4G I see:
> 1. As Gerd pointed out it is also used to setup a part of e820 tables.
>    It possibly could be also changed see "[Qemu-devel] [RfC PATCH] e820: pass
>    high memory too" but it will be more complicated to keep seabios
>    backward/forward compatible if we use RamSizeOver4G for passing.

What are the compatibility worries, exactly?

The way I see it, if QEMU wants to be compatible it can't
use memory hotplug. In that case RamSizeOver4G will have
the old meaning which should work without issues.

> 2. It doesn't scale if we are going to use more than 1Tb future.
>    Extending RamSizeOver4G value via additional CMOS port wasn't welcomed
>    either, see
> http://www.seabios.org/pipermail/seabios/2010-September/000911.html
> 
> It looks like fw_cfg interface is preferred one and "etc/pcimem64-start"
> approach doesn't have backward/forward compatibility issues.

Hmm but you aren't passing in memory size - pcimem64-start
merely tells bios where to put PCI devices.
So how would it help?

> > 
> > 
> > > ---
> > >  src/fw/pciinit.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/src/fw/pciinit.c b/src/fw/pciinit.c
> > > index b29db99..798309b 100644
> > > --- a/src/fw/pciinit.c
> > > +++ b/src/fw/pciinit.c
> > > @@ -18,6 +18,8 @@
> > >  #include "paravirt.h" // RamSize
> > >  #include "string.h" // memset
> > >  #include "util.h" // pci_setup
> > > +#include "byteorder.h" // le64_to_cpu
> > > +#include "romfile.h" // romfile_loadint
> > >  
> > >  #define PCI_DEVICE_MEM_MIN     0x1000
> > >  #define PCI_BRIDGE_IO_MIN      0x1000
> > > @@ -764,6 +766,15 @@ static void pci_bios_map_devices(struct pci_bus *busses)
> > >  {
> > >      if (pci_bios_init_root_regions(busses)) {
> > >          struct pci_region r64_mem, r64_pref;
> > > +        u64 ram64_end = 0x100000000ULL + RamSizeOver4G;
> > > +        u64 base64 = le64_to_cpu(romfile_loadint("etc/pcimem64-start",
> > > +                                 ram64_end));
> > > +        if (base64 < ram64_end) {
> > > +            dprintf(1, "ignorig etc/pcimem64-start [0x%llx] below present RAM, "
> > > +                       "placing 64-bit PCI window behind RAM end: %llx",
> > > +                        base64, ram64_end);
> > > +            base64 = ram64_end;
> > > +        }
> > >          r64_mem.list.first = NULL;
> > >          r64_pref.list.first = NULL;
> > >          pci_region_migrate_64bit_entries(&busses[0].r[PCI_REGION_TYPE_MEM],
> > > @@ -779,7 +790,7 @@ static void pci_bios_map_devices(struct pci_bus *busses)
> > >          u64 align_mem = pci_region_align(&r64_mem);
> > >          u64 align_pref = pci_region_align(&r64_pref);
> > >  
> > > -        r64_mem.base = ALIGN(0x100000000LL + RamSizeOver4G, align_mem);
> > > +        r64_mem.base = ALIGN(base64, align_mem);
> > >          r64_pref.base = ALIGN(r64_mem.base + sum_mem, align_pref);
> > >          pcimem64_start = r64_mem.base;
> > >          pcimem64_end = r64_pref.base + sum_pref;
> > > -- 
> > > 1.8.3.1
> > 
> 
> 
> -- 
> Regards,
>   Igor