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Re: [Qemu-devel] [PATCH] linux-user EFAULT implementation

From: Fabrice Bellard
Subject: Re: [Qemu-devel] [PATCH] linux-user EFAULT implementation
Date: Tue, 10 Jul 2007 22:45:16 +0200
User-agent: Thunderbird (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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