hi, > On Thu, Oct 28, 2010 at 05:31:45AM +0000, YAMAMOTO Takashi wrote: >> hi, >> >> > Here's the reason why I've written xmd_machdep.c: >> > >> > xmd(4) is a read-only RAM-based disk driver capable of XIP. The >> > main purpose is to test XIP functionality. xmd(4) can be implemented >> > on any platforms that supports VM in theory. xmd(4) may be also >> > useful for other cases where md(4) is used, but users want to save >> > memory. md(4) allocates memory for its storage, and copies pages >> > from or to page cache. >> > >> > xmd(4) allocates a static, read-only array and provides it as a >> > backing store. When it's used as XIP, it registers the array as >> > a physical device page segment. From VM's POV, the registered >> > region is seen like a ROM in a device connected over some buses. >> > >> > The procedure to register an array as a physical segment is somewhat >> > strange. The registered array resides in kernel's read-only data >> > section. Kernel already maps its static region (text, rodata, >> > data, bss, ...) at boot time. xmd(4) "re-defines" part of it as >> > a physical device segment, like a ROM connected via another bus. >> > >> > As far as the backing store array resides in main memory, you'll >> > end up with some way to convert kernel VA back to physical address. >> > There is no alternative to achieve the goal in MI way, or xmd.c is >> > sprinkled like mem.c. >> >> why can't you use pmap_extract? > > Because looking up a paddr_t doesn't help alone. > > The driver needs to allocate a physical segment object (struct > vm_physseg) and per-page objects (struct vm_page), so that its > region can be mapped to user address. This is done by calling > bus_space_physload_device() or xmd_machdep_physload(), which in > turn call uvm_page_physload_device(). > > This is what happens during a fault onto xmd: > > - User opens a cdev (/dev/XXX), then calls mmap() with its fd > - User touch a mapped address > - Fault is triggered, fault handler looks up user's map and map > entry > - uvm_fault() -> udv_fault() -> cdev_mmap() -> xmd_mmap() > - xmd_mmap() returns a "paddr_t" > - udv_fault() enters the paddr_t to pmap_enter() > - pmap_enter looks up a vm_physseg from a paddr_t > - pmap_enter looks up a vm_page from a vm_physseg > - pmap_enter looks up a vm_page_md from a vm_page > : > > This is redundant. The problem is we use "paddr_t" as a cookie > to identify a page in a segment, overriding its original meaning, > physical address. What pmap_enter needs is an ID. Looking up a > physical address from an ID is easy. The reverse is not. > > After these observations, I have concluded that any appearance of > "paddr_t" in any MI code (sys/uvm, sys/kern, sys/dev) is a wrong > approach. I don't see how pmap_extract() helps this situation?
because you seem saying that there is no MI way to "convert kernel VA back to physical address", i suggested pmap_extract. i guess i don't understand your situation. :-) YAMAMOTO Takashi > > Masao > >> >> YAMAMOTO Takashi >> >> > >> > Masao > > -- > Masao Uebayashi / Tombi Inc. / Tel: +81-90-9141-4635
