Re: xmd(4) (Re: XIP)

[Masao Uebayashi <uebayasi%tombi.co.jp@localhost>]

On Thu, Oct 28, 2010 at 07:59:17AM +0000, YAMAMOTO Takashi wrote:
> 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. :-)

I come to think that pmap_extract(9) API is unnecessary at all.
See other mails for the details...

Masao

> 
> YAMAMOTO Takashi
> 
> > 
> > Masao
> > 
> >> 
> >> YAMAMOTO Takashi
> >> 
> >> > 
> >> > Masao
> > 
> > -- 
> > Masao Uebayashi / Tombi Inc. / Tel: +81-90-9141-4635

-- 
Masao Uebayashi / Tombi Inc. / Tel: +81-90-9141-4635