|Subject:||Re: [Qemu-devel] [PATCH] linux-user EFAULT implementation|
|Date:||Tue, 10 Jul 2007 22:45:16 +0200|
|User-agent:||Thunderbird 220.127.116.11 (X11/20070212)|
Paul Brook wrote: (...]
Using g2h directly is bad. g2h is an implementation detail of one particular memory model.The whole point of the lock_user abstraction (or a similar copy_from_user abstraction) is that almost none of the code cares how "user" memory is accessed. One of the long-term goals of this abstraction is to allow the softmmu code to be used with userspace emulation. In this case a region may be split across multiple discontiguous host pages.
I confirm that I expected g2h() to be completely removed in case the Linux user access API is used.
The reason I used a locking paradigm rather than a copying one is that it allows a zero-copy implementation in the common case. I've no strong objections to a copying interface, however it must be implementation agnostic.
Since you agree for the copying interface, I suggest to convert all the code to it. The exact implementation of access_ok(), copy_to_user(), copy_from_user()... will be easily modifiable if one day softmmu user mode is needed.
In the only specific case of potentially big unbounded memory areas (such as read/write buffers), a different API must be used where it is possible to get a pointer to user pages (e.g. ptr = lock_user_page(addr, rw_mode); if (!ptr) return -EFAULT; unlock_user_page(ptr)), so that no copy is performed while being able to handle any user page remapping.
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