Message-ID: <20250709112201.GA26241@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Wed, 9 Jul 2025 04:22:01 -0700
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: Naman Jain <namjain@...ux.microsoft.com>
Cc: "K . Y . Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Long Li <longli@...rosoft.com>,
	Michael Kelley <mhklinux@...look.com>, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org, Olaf Hering <olaf@...fle.de>
Subject: Re: [PATCH v3] tools/hv: fcopy: Fix irregularities with size of ring
 buffer

On Tue, Jul 08, 2025 at 01:33:19PM +0530, Naman Jain wrote:
> Size of ring buffer, as defined in uio_hv_generic driver, is no longer
> fixed to 16 KB. This creates a problem in fcopy, since this size was
> hardcoded. With the change in place to make ring sysfs node actually
> reflect the size of underlying ring buffer, it is safe to get the size
> of ring sysfs file and use it for ring buffer size in fcopy daemon.
> Fix the issue of disparity in ring buffer size, by making it dynamic
> in fcopy uio daemon.
> 
> Cc: stable@...r.kernel.org
> Fixes: 0315fef2aff9 ("uio_hv_generic: Align ring size to system page")
> Signed-off-by: Naman Jain <namjain@...ux.microsoft.com>
> ---
>  tools/hv/hv_fcopy_uio_daemon.c | 82 +++++++++++++++++++++++++++++++---
>  1 file changed, 75 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/hv/hv_fcopy_uio_daemon.c b/tools/hv/hv_fcopy_uio_daemon.c
> index 0198321d14a2..5388ee1ebf4d 100644
> --- a/tools/hv/hv_fcopy_uio_daemon.c
> +++ b/tools/hv/hv_fcopy_uio_daemon.c
> @@ -36,6 +36,7 @@
>  #define WIN8_SRV_VERSION	(WIN8_SRV_MAJOR << 16 | WIN8_SRV_MINOR)
>  
>  #define FCOPY_UIO		"/sys/bus/vmbus/devices/eb765408-105f-49b6-b4aa-c123b64d17d4/uio"
> +#define FCOPY_CHANNELS_PATH	"/sys/bus/vmbus/devices/eb765408-105f-49b6-b4aa-c123b64d17d4/channels"

We can use a single path up to the device ID and then append either 'uio' or 'channels' using
two separate variables.

>  
>  #define FCOPY_VER_COUNT		1
>  static const int fcopy_versions[] = {
> @@ -47,9 +48,62 @@ static const int fw_versions[] = {
>  	UTIL_FW_VERSION
>  };
>  
> -#define HV_RING_SIZE		0x4000 /* 16KB ring buffer size */
> +static uint32_t get_ring_buffer_size(void)
> +{
> +	char ring_path[PATH_MAX];
> +	DIR *dir;
> +	struct dirent *entry;
> +	struct stat st;
> +	uint32_t ring_size = 0;
> +	int retry_count = 0;
>  
> -static unsigned char desc[HV_RING_SIZE];
> +	/* Find the channel directory */
> +	dir = opendir(FCOPY_CHANNELS_PATH);
> +	if (!dir) {
> +		usleep(100 * 1000); /* Avoid race with kernel, wait 100ms and retry once */
> +		dir = opendir(FCOPY_CHANNELS_PATH);
> +		if (!dir) {
> +			syslog(LOG_ERR, "Failed to open channels directory: %s", strerror(errno));
> +			return 0;
> +		}
> +	}
> +
> +retry_once:
> +	while ((entry = readdir(dir)) != NULL) {
> +		if (entry->d_type == DT_DIR && strcmp(entry->d_name, ".") != 0 &&
> +		    strcmp(entry->d_name, "..") != 0) {
> +			snprintf(ring_path, sizeof(ring_path), "%s/%s/ring",
> +				 FCOPY_CHANNELS_PATH, entry->d_name);
> +
> +			if (stat(ring_path, &st) == 0) {
> +				/*
> +				 * stat returns size of Tx, Rx rings combined,
> +				 * so take half of it for individual ring size.
> +				 */
> +				ring_size = (uint32_t)st.st_size / 2;
> +				syslog(LOG_INFO, "Ring buffer size from %s: %u bytes",
> +				       ring_path, ring_size);
> +				break;
> +			}
> +		}
> +	}
> +
> +	if (!ring_size && retry_count == 0) {
> +		retry_count = 1;
> +		rewinddir(dir);
> +		usleep(100 * 1000); /* Wait 100ms and retry once */
> +		goto retry_once;

		Is this retry solving any real problem ?

> +	}
> +
> +	closedir(dir);
> +
> +	if (!ring_size)
> +		syslog(LOG_ERR, "Could not determine ring size");
> +
> +	return ring_size;
> +}
> +
> +static unsigned char *desc;
>  
>  static int target_fd;
>  static char target_fname[PATH_MAX];
> @@ -406,7 +460,7 @@ int main(int argc, char *argv[])
>  	int daemonize = 1, long_index = 0, opt, ret = -EINVAL;
>  	struct vmbus_br txbr, rxbr;
>  	void *ring;
> -	uint32_t len = HV_RING_SIZE;
> +	uint32_t ring_size, len;
>  	char uio_name[NAME_MAX] = {0};
>  	char uio_dev_path[PATH_MAX] = {0};
>  
> @@ -437,6 +491,20 @@ int main(int argc, char *argv[])
>  	openlog("HV_UIO_FCOPY", 0, LOG_USER);
>  	syslog(LOG_INFO, "starting; pid is:%d", getpid());
>  
> +	ring_size = get_ring_buffer_size();
> +	if (!ring_size) {
> +		ret = -ENODEV;
> +		goto exit;
> +	}
> +
> +	len = ring_size;

	Do we need this ?

> +	desc = malloc(ring_size * sizeof(unsigned char));
> +	if (!desc) {
> +		syslog(LOG_ERR, "malloc failed for desc buffer");
> +		ret = -ENOMEM;
> +		goto exit;
> +	}

	This memory is not being freed anywhere. While I agree that freeing memory at
	program exit may not have much practical value, we can easily address
	this by adding a goto label for cleanup, this will keep all the static code
	analyzers happy.

> +
>  	fcopy_get_first_folder(FCOPY_UIO, uio_name);
>  	snprintf(uio_dev_path, sizeof(uio_dev_path), "/dev/%s", uio_name);
>  	fcopy_fd = open(uio_dev_path, O_RDWR);
> @@ -448,14 +516,14 @@ int main(int argc, char *argv[])
>  		goto exit;
>  	}
>  
> -	ring = vmbus_uio_map(&fcopy_fd, HV_RING_SIZE);
> +	ring = vmbus_uio_map(&fcopy_fd, ring_size);
>  	if (!ring) {
>  		ret = errno;
>  		syslog(LOG_ERR, "mmap ringbuffer failed; error: %d %s", ret, strerror(ret));
>  		goto close;
>  	}
> -	vmbus_br_setup(&txbr, ring, HV_RING_SIZE);
> -	vmbus_br_setup(&rxbr, (char *)ring + HV_RING_SIZE, HV_RING_SIZE);
> +	vmbus_br_setup(&txbr, ring, ring_size);
> +	vmbus_br_setup(&rxbr, (char *)ring + ring_size, ring_size);
>  
>  	rxbr.vbr->imask = 0;
>  
> @@ -472,7 +540,7 @@ int main(int argc, char *argv[])
>  			goto close;
>  		}
>  
> -		len = HV_RING_SIZE;
> +		len = ring_size;
>  		ret = rte_vmbus_chan_recv_raw(&rxbr, desc, &len);
>  		if (unlikely(ret <= 0)) {
>  			/* This indicates a failure to communicate (or worse) */
> 
> base-commit: 26ffb3d6f02cd0935fb9fa3db897767beee1cb2a
> -- 
> 2.34.1

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