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SubjectRe: [patch 02/12] syslets: add syslet.h include file, user API/ABI definitions

* Kevin O'Connor <kevin@koconnor.net> wrote:

> I'd like to propose a simpler userspace API for syslets. I believe
> this revised API is just as capable as yours (anything done purely in
> kernel space with the existing API can also be done with this one).
>
> An "atom" would look like:
>
> struct syslet_uatom {
> u32 nr;
> u64 ret_ptr;
> u64 next;
> u64 arg_nr;
> u64 args[6];
> };
>
> The sys_nr, ret_ptr, and next fields would be unchanged. The args
> array would directly store the arguments to the system call. To
> optimize the case where only a few arguments are necessary, an
> explicit argument count would be set in the arg_nr field.

i dont see the advantage of arg_nr - if the arguments are direct then
the best way is to just fetch them all, not to do testing upon arg_nr.
Furthermore, regarding the indirect pointers, they are quite essential
for some uses, see:

http://lkml.org/lkml/2007/2/28/292

> Of course, the above lacks the syscall return testing capabilities in
> your atoms. To obtain that capability, one could add a new syscall:
>
> long sys_syslet_helper(long flags, long *ptr, long inc, u64 new_next)

yes. But a 'flags' field is handy anyway, to signal things like
NOCOMPLETE or SYSLET_SYNC/SYSLET_ASYNC (a flags field is always useful
in such structures). So the condition testing comes 'for free' in
essence.

but ... as you can see it with sys_umem_add(), i like the addition of
helper syscalls - i just think that this particular one wouldnt be too
helpful.

> I would also change the event ring notification system. Instead of
> building that support into all syslets, one could introduce an "add to
> head" syscall specifically for that purpose. If done this way,
> userspace could arrange for this new sys_addtoring call to always be
> the last uatom executed in a syslet. This would make the support
> optional - those userspace applications that prefer to use a futex or
> signal as an event system could arrange to have those system calls as
> the last one in the chain instead. [...]

the problem is signals: a syslet has to return to user-space upon
signals or upon a stop condition. So to notify about the precise place
of stoppage, the notification ring is a 'built in' property.

(nevertheless, as i mentioned it in a prior mail, i'll create separate
ring syscalls, because they are useful for other stuff too.)

> For example, the open & stat case could be done with a chain like the
> following:
>
> atom1: &atom3->args[1] = sys_open(...)
> atom2: sys_syslet_helper(SYSLET_BRANCH_ON_NON_POSITIVE,
> &atom3->args[1], 0, atom4)

i dont see a huge conceptual difference between having the syslet helper
in flags versus having it in a separate syscall. Other than yours has
twice the number of atoms.

> It is also possible to use sys_syslet_helper to push a return value to
> multiple syslet parameters (for example, propagating an fd from open
> to multiple reads). For example:
>
> atom1: &atom3->args[1] = sys_open(...)
> atom2: &atom4->args[1] = sys_syslet_helper(0, &atom3->args[1], 0, 0)
> atom3: sys_read([arg1 filled in atom1], ...)
> atom4: sys_read([arg1 filled in atom2], ...)

try to do this in FIO. You'd have to create many extra atoms to push the
fd into the argument fields - instead of just sharing the variable.
Sharing is /good/. These 'simplifications' complicate the whole syslet
programming model down to being near unusable.

> Although this is a bit ugly, I must wonder how many times one would
> build chains complex enough to require it.

take a look at FIO.

Ingo
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