Re: [PATCH] misc: fastrpc: copy to user only for non-DMA-BUF heap buffers

[jeyr]

On 2021-09-23 23:13, Greg KH wrote:
> On Thu, Sep 23, 2021 at 03:54:15PM +0530, jeyr@xxxxxxxxxxxxxx wrote:
> > On 2021-09-23 14:19, Greg KH wrote:
> > > On Thu, Sep 23, 2021 at 02:07:52PM +0530, Jeya R wrote:
> > > > fastrpc_put_args is copying all the output buffers to user. For large
> > > > number of output context buffers, this might cause performance
> > > > degradation. Copying is not needed for DMA-BUF heap buffers.
> > >
> > > What does "performance degradation" really mean?
> >
> > Unnecessary copying for large number of buffers would cause some
> > additional time which would get added to overall fastrpc call cost.
> >
> > >
> > > >
> > > > Signed-off-by: Jeya R <jeyr@xxxxxxxxxxxxxx>
> > > > ---
> > > >  drivers/misc/fastrpc.c | 18 ++++++++++--------
> > > >  1 file changed, 10 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> > > > index beda610..536eabf 100644
> > > > --- a/drivers/misc/fastrpc.c
> > > > +++ b/drivers/misc/fastrpc.c
> > > > @@ -890,15 +890,17 @@ static int fastrpc_put_args(struct
> > > > fastrpc_invoke_ctx *ctx,
> > > >  	inbufs = REMOTE_SCALARS_INBUFS(ctx->sc);
> > > >
> > > >  	for (i = inbufs; i < ctx->nbufs; ++i) {
> > > > -		void *src = (void *)(uintptr_t)rpra[i].pv;
> > > > -		void *dst = (void *)(uintptr_t)ctx->args[i].ptr;
> > > > -		u64 len = rpra[i].len;
> > > > +		if (!ctx->maps[i]) {
> > > > +			void *src = (void *)(uintptr_t)rpra[i].pv;
> > > > +			void *dst = (void *)(uintptr_t)ctx->args[i].ptr;
> > >
> > > uintptr_t is not a kernel variable type.  Please use the real kernel
> > > type for this as you are touching these lines.
> > >
> >
> > Sure, thanks for pointing this. Will update this in the next patch.
> >
> > > > +			u64 len = rpra[i].len;
> > > >
> > > > -		if (!kernel) {
> > > > -			if (copy_to_user((void __user *)dst, src, len))
> > > > -				return -EFAULT;
> > > > -		} else {
> > > > -			memcpy(dst, src, len);
> > > > +			if (!kernel) {
> > > > +				if (copy_to_user((void __user *)dst, src, len))
> > > > +					return -EFAULT;
> > > > +			} else {
> > > > +				memcpy(dst, src, len);
> > > > +			}
> > >
> > > So you were copying buffers that didn't need to be copied?  So you are
> > > now doing less work?  Or is this fixing a bug where you were copying
> > > things that you should not have been copying?
> > >
> > > What commit does this fix?  Does this need to go to the stable kernel
> > > trees?
> > >
> >
> > Yes, not all buffer needs to be copied. This change would avoid 
> > unnecessary
> > copying of buffers. Not adding fix tag as it's not exactly fixing any 
> > bug.
> > This should go to stable kernel trees.
>
> If it's not fixing a bug, why should it go into the stable trees?

This is not a bug fix, it can be considered as an enhancement and it can 
go in
to new release.