Re: [Qemu-devel] [sneak preview] major scsi overhaul

[Gerd Hoffmann <kraxel@redhat.com>]

On 11/09/09 13:37, Avi Kivity wrote:
> On 11/09/2009 11:08 AM, Gerd Hoffmann wrote:
>> It is because of QEMUSGList which drags in a target_phys_addr_t
>> dependency.
>
> As Michael notes, devices have physical address sizes independent of the
> target platform; a PCI device that supports 64-bit addresses can be
> plugged into a motherboard that supports 32-bit address bus processors.
>
> We can fix this in several ways:
> - creating QEMUSG64List and QEMUSG32List (and typedefing PCISGList to
> the former)

Doesn't fly.  Either every SCSI HBA uses QEMUSG64List (thus making this 
aequivalent to the next option from the scsi point of view), or 
scsi-disk would have to support both formats.

> - making QEMUSGList always use 64-bit addresses since it will almost
> always be used with devices (which are often 64-bit capable)

Possible.

> - making target_phys_addr_t always 64-bit (which loses some performance
> with 32-on-32 emulation)

Possible too.

Has the advantage that more code can be compiled only once, often 
target_phys_addr_t is the only reason a source file is in Makefile.hw 
instead of Makefile.

Question is how big the performance hit actually is and whenever we are 
willing to accept that or not ...

cheers,
   Gerd