Re: [Qemu-devel] [RFC v1 19/22] memory: per-AddressSpace dispatch

[Avi Kivity <avi@redhat.com>]

On 10/04/2012 10:47 AM, Peter Maydell wrote:
>>> I'd make address_space_* use uint64_t instead of target_phys_addr_t
>>> for the address. It may actually be buggy for 32 bit
>>> target_phys_addr_t  and 64 bit DMA addresses, if such architectures
>>> exist. Maybe memory.c could be made target independent one day.
> 
> I thought target_phys_addr_t was supposed to be "widest necessary
> address", so an architecture with 64 bit DMA addresses should be
> implemented with 64 bit target_phys_addr_t...

Yes.  Since even before this patchset all DMA went through the memory
API (and before the memory API, through cpu_register_physical_memory),
this case was already broken.

> 
>> We can make target_phys_addr_t 64 bit unconditionally.  The fraction of
>> deployments where both host and guest are 32 bits is dropping, and I
>> doubt the performance drop is noticable.
> 
> I benchmarked it on ARM when I switched ARM to always 64 bit physaddrs;
> it was just about measurable (maybe half a percent or so) but not
> significant (we don't use this type in the fast path for memory
> accesses, only in the slow/IO path).
> 
> I'd favour just moving everything to 64 bits (the remaining
> 32 bit targets are: cris, lm32, m68k, microblaze, or32, sh4, unicore32,
> xtensa). However it does require some review of devices to check that
> they're not using target_phys_addr_t when they meant uint32_t [eg
> in registers for DMA devices] or relying on 32 bit wraparound.

I posted a patch.  I did no review, the cost/benefit tradeoff is
horrible IMO, especially with me not knowing the details.  If something
breaks, git bisect will save the day.

-- 
error compiling committee.c: too many arguments to function