Re: [SeaBIOS] [Qemu-devel] XP install cores with SCSI LSI 53C895A disks

[Gerhard Wiesinger]

On Thu, 8 Mar 2012, Gerd Hoffmann wrote:
> On 03/07/12 20:58, Gerhard Wiesinger wrote:
> > On Wed, 7 Mar 2012, Brian Jackson wrote:
> > > I think most people trying to use qemu for anything useful have given
> > > up on if=scsi. Some distros even disable support because they don't
> > > want to QA it. That should be a decent sign that you may want to avoid
> > > it.
> >
> > OK, but SAS (Serial attached SCSI) is technology in the area of storage
> > interface technology where all big storage vendors see future (e.g. they
> > give up: FC and SATA drives, SATA drives are replaced by MDL SATA drives
> > (SATA 7200RPM drives with SAS interface)).
>
> The problem isn't scsi.  The problem is the lsi adapter.  Problem #1 is
> the hardware design which makes it hard to emulate it correctly and #2
> that you need a non-redistributable rom file to boot from it.

Advantages of LSI 53C895A over others:
1.) OS support is great, even for legacy systems: DOS, Win 3.1, Win95, NT 
4, W2K, XP, Vista, Win7, Linux, etc. I don't know any adapter with such 
wide range of OS support. Also tested up to 2TB of LUNs.
2.) OS support out of the box without additional drivers for a lot of 
newer OSes
3.) Migration from VMWare SCSI is easy
4.) Works well here for about 1 year for legacy VMs for DOS and NT 4
5.) BIOS geometry translation is correct, also according to 
partition table (doesn't work correctly on MEGASAS,
INT13 geometry interrupts are not correct, already reported to 
Hannes)
7.) LSI SCSI DMA technology is fast. I'm getting /dev/null performance 
over 500MB/s ..., optimized for parallel IOPS, etc.
8.) Faster ROM init than MEGASAS

Disadvantages:
1.) ROM non distributable
2.) non complete implementation
3.) 2TB limit
4.) Slower ROM INIT than without any Option ROM

Megasas also needs a ROM rom to boot from it.
Not to forget iSCSI which is also current/future SCSI technology ...

Even if it is hard but I think the goal of qemu should be to implement all 
supported hardware pieces as good as possible (e.g. LSI 53C895A). I know 
it is not an easy task (see my thread about rtl8139) but I think together 
we can manage it.

Are there any LSI 53C895A known bugs or incomplete implementation issues 
which are known? 
Any hints on the core dump?
> > Therefore I don't understand why distros are giving up SAS which is also
> > SCSI (of course old legacy SCSI is understandable).
>
> Nobody gives up on scsi.  See virtio-scsi merged recently.  There also
> is megasas aiming for merge (which shares the boot issue with lsi though).

Disadvantage of virtio-scsi is that drivers are needed.
Is INT13h supported for legacy OS and to boot? Future? But as far as I saw 
implemented in seabios, right?
What BIOS translation is used? Buslogic? LSI logic? Partition guessing?

BTW: I've found out how LSI logic BIOS geometry translation works with 
(guessing) and without existing partition table. That might be an option
for Seabios to implement.
@Kevin: What do you think?

Ciao,
Gerhard

[Qemu-devel] XP install cores with SCSI LSI 53C895A disks

[Gerd Hoffmann]

Hi,
> Advantages of LSI 53C895A over others:
> 1.) OS support is great, even for legacy systems: DOS, Win 3.1, Win95,
> NT 4, W2K, XP, Vista, Win7, Linux, etc. I don't know any adapter with
> such wide range of OS support. Also tested up to 2TB of LUNs.

Hmm?  win7 here?  At least the 64bit versions don't ship lsi drivers
(also other oldish hardware such as ac97 sound).  Not sure about the
32bit versions.

legacy systems is *the* strong argument for lsi.
> 2.) OS support out of the box without additional drivers for a lot of
> newer OSes

recent windows systems stopped shipping lsi drivers.
linux systems will ship with virtio-scsi support soon.
> 7.) LSI SCSI DMA technology is fast. I'm getting /dev/null performance
> over 500MB/s ..., optimized for parallel IOPS, etc.

Well, you can't do zerocopy because that is next to impossible to
emulate correctly.  Which puts lsi behind other solutions
performance-wise.  But to keep legacy systems alive performance isn't
the #1 issue.  Likewise the 2T limit ;)
> Not to forget iSCSI which is also current/future SCSI technology ...

Oh, booting from iscsi is supported just fine by iPXE.  It is more a
problem that the guest OS must be able to handle this too ...

cheers,
  Gerd

XP install cores with SCSI LSI 53C895A disks

[Paolo Bonzini]

Il 09/03/2012 08:35, Gerd Hoffmann ha scritto:
> Is INT13h supported for legacy OS and to boot? Future? But as far as
> I saw implemented in seabios, right?

Yes, Kevin (O' Connor, not Wolf) integrated the patch this week.  It
will also be in Fedora 17.
> What BIOS translation is used? Buslogic? LSI logic? Partition guessing?

Whatever QEMU does for IDE.  It is passed down via mode page 04h and
retrieved by SeaBIOS.

Paolo

[Qemu-devel] XP install cores with SCSI LSI 53C895A disks

[Kevin O'Connor]

On Fri, Mar 09, 2012 at 07:18:38AM +0100, Gerhard Wiesinger wrote:
> BTW: I've found out how LSI logic BIOS geometry translation works
> with (guessing) and without existing partition table. That might be
> an option
> for Seabios to implement.
> @Kevin: What do you think?

SeaBIOS has an algorithm for guessing the virtual disk geometry (see
src/block.c:get_translation).  If you have an improvement to that
algorithm, please send a patch.

Note that the physical disk geometry is obtained from the drive itself
(where possible).

-Kevin