Re: [Qemu-devel] [PATCH 0/4] introduce cpu_physical_memory_map_fast

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/06/2011 07:27 AM, Paolo Bonzini wrote:
> On 05/31/2011 11:16 AM, Paolo Bonzini wrote:
>> On 05/12/2011 04:51 PM, Paolo Bonzini wrote:
>>> On 05/03/2011 06:49 PM, Paolo Bonzini wrote:
>>>> Paravirtualized devices (and also some real devices) can assume they
>>>> are going to access RAM. For this reason, provide a fast-path
>>>> function with the following properties:
>>>>
>>>> 1) it will never allocate a bounce buffer
>>>>
>>>> 2) it can be used for read-modify-write operations
>>>>
>>>> 3) unlike qemu_get_ram_ptr, it is safe because it recognizes "short"
>>>> blocks
>>>>
>>>> Patches 3 and 4 use this function for virtio devices and the milkymist
>>>> GPU. The latter is only compile-tested.
>>>>
>>>> Another function checks if it is possible to split a contiguous
>>>> physical
>>>> address range into multiple subranges, all of which use the fast path.
>>>> I will introduce later a use for this function.
>>>>
>>>> Paolo Bonzini (4):
>>>> exec: extract cpu_physical_memory_map_internal
>>>> exec: introduce cpu_physical_memory_map_fast and
>>>> cpu_physical_memory_map_check
>>>> virtio: use cpu_physical_memory_map_fast
>>>> milkymist: use cpu_physical_memory_map_fast
>>>>
>>>> cpu-common.h | 4 ++
>>>> exec.c | 108 +++++++++++++++++++++++++++++++++++++-------------
>>>> hw/milkymist-tmu2.c | 39 ++++++++++--------
>>>> hw/vhost.c | 10 ++--
>>>> hw/virtio.c | 2 +-
>>>> 5 files changed, 111 insertions(+), 52 deletions(-)
>>>>
>>>
>>> Ping?
>>
>> Ping^2?
>
> Ping^3?

Oh, the patch series basically died for me when I saw:

Avi> What performance benefit does this bring?

Paolo> Zero

Especially given Avi's efforts to introduce a new RAM API, I don't want 
yet another special case to handle.

You're just trying to avoid having to handle map failures, right?  So 
it's not really cpu_physical_memory_map_fast, it's really 
cpu_physical_memory_map_simple?

I'd prefer that a device just treat failures as fatal vs. using a new API.

Regards,

Anthony Liguori

> Paolo
>