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

From: jeyr
Date: Thu Sep 23 2021 - 06:24:46 EST


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.

Thanks

thanks,

greg k-h