Blaisorblade wrote:
On Monday 16 January 2006 20:34, Jacob Bachmeyer wrote:
Has any thought been given to making SKAS4 suitably generic that it
could be used for more than just UML?
Not yet, thoughts welcome.
Let's see:
to support HURD (which uses the Mach ABI):
-- existing facilities plus trap lcall gates
to support WINE (which follows Win32 conventions (ick!)): (x86 only)
--existing facilities plus
-- trap on access to specified pages
Explanation: Win32 API calls are not syscalls in the normal
sense--rather they are made by calling into a system DLL. These DLLs
are mapped into the process' address space on Windows and under current
WINE, much like shared objects in normal Linux. This idea would enable
WINE to not actually map these DLLs, but rather simply set the pages
where the DLLs would be mapped as "fasttrap". Then, when the program
attempts to access a DLL's memory image, the kernel would intercept the
request and quickly pass it to a userspace thread, which handles the
"page fault". The page remains set as "fasttrap", and the control
process modifies the address space and CPU context appropriately before
allowing execution to continue.
-- read/write in guest address space
Explanation: mmap is fine for big changes to an address space
(such as loading modules), but one capability WINE would need for this
to be truly useful is 1/2/4/8/16-byte PEEK and POKE. (Some Win32
programs like to do wierd things with Windows' system code--in
conjunction with "fasttrap", this would allow WINE to keep such programs
happy.) As I understand, ptrace already provides this, hopefully
adequetely.
-- intercept arbitrary interrupts in guest address space
Explanation: Many older Windows programs (Win16 era)
occasionally directly invoke various soft interrupts (these are
basically DOS syscalls). The ability to intercept these is necessary,
but need not be particularly efficient or fast.
-- modify guest address space's LDT
Explanation: Again, Win16 support. Old Windows actually
allowed processes to request segments for whatever purpose. This may or
may not be doable on all modern hardware.
-- transparently use threads in guest address spaces, if desired
Explanation: WINE currently uses the host's scheduler.
Changing it to this new API shouldn't adversely affect that ability.
(And on second thought, using a UML library might not be an option.)
PS: If I understand correctly, UML with the current SKAS3 works by
swapping processes into and out of a single "user" address space.
I
propose a system where many distinct "user" address spaces are
maintained by the kernel and execution is placed whereever the user-mode
scheduler says.
What you say is not clear, but the most obvious understanding of the above
sentence is that you propose what already happens.
However, SKAS3 and current ideas for SKAS4, with different APIs but similar
semantics, say: implement all guest processes as user-level threads (totally
implemented within UML) with the exception that we allow different address
spaces.
So we have "switch the guest proc to a different address
space" (PTRACE_SWITCH_MM, in arch/i386/kernel/ptrace.c), manipulate with
mmap/munmap/mprotect any of these address spaces, and destroy it (all in
mm/proc_mm.c).
I shall clarify my proposal: each thread is assigned an address space,
while an address space can contain multiple threads. Each thread also
has a STOP/RUN flag, which if set to RUN, causes the host scheduler to
consider that thread for execution (along with all other runnable
threads). This flag allows either the userspace control process to make
scheduling decisions itself, (by only setting one of its threads to RUN)
or to punt and have the kernel handle all scheduling for its threads (by
setting them all to RUN and using STOP only to block a thread).
Could all SKAS4 APIs be multiplexed through one syscall? (Perhaps
simply as more ptrace functions, or as a new "skas4" syscall?)
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