Re: [PATCH v6 12/17] powerpc/pseries/vas: Integrate API with open/close windows

[Haren Myneni <haren@linux.ibm.com>]

On Fri, 2021-06-18 at 09:22 +1000, Nicholas Piggin wrote:
> Excerpts from Haren Myneni's message of June 18, 2021 6:36 am:
> > This patch adds VAS window allocatioa/close with the corresponding
> > hcalls. Also changes to integrate with the existing user space VAS
> > API and provide register/unregister functions to NX pseries driver.
> > 
> > The driver register function is used to create the user space
> > interface (/dev/crypto/nx-gzip) and unregister to remove this
> > entry.
> > 
> > The user space process opens this device node and makes an ioctl
> > to allocate VAS window. The close interface is used to deallocate
> > window.
> > 
> > Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> 
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> 
> Unless there is some significant performance reason it might be
> simplest
> to take the mutex for the duration of the allocate and frees rather
> than 
> taking it several times, covering the atomic with the lock instead.
> 
> You have a big lock, might as well use it and not have to wonder what
> if 
> things race here or there.

Using mutex to protect allocate/deallocate window and setup/free IRQ,
also to protect updating the list. We do not need lock for modify
window hcall and other things. Hence taking mutex several times. Also
used atomic for counters (used_lpar_creds) which can be exported in
sysfs (this patch will be added later in next enhancement seris). 

Genarlly applications open window initially, do continuous copy/paste
operations and close window later. But possible that the library /
application to open/close window for each request. Also may be opening
or closing multiple windows (say 1000 depends on cores on the system)
at the same time. These cases may affect the application performance.

Thanks
Haren

> 
> But don't rework that now, maybe just something to consider for
> later.
> 
> Thanks,
> Nick
>