linux-kernel - Re: [RFC Patch 08/12] IXGBEVF: Rework code of finding the end transmit desc of package

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Date:	Thu, 22 Oct 2015 15:58:55 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Lan Tianyu <tianyu.lan@...el.com>
Cc:	bhelgaas@...gle.com, carolyn.wyborny@...el.com,
	donald.c.skidmore@...el.com, eddie.dong@...el.com,
	nrupal.jani@...el.com, yang.z.zhang@...el.com, agraf@...e.de,
	kvm@...r.kernel.org, pbonzini@...hat.com, qemu-devel@...gnu.org,
	emil.s.tantilov@...el.com, intel-wired-lan@...ts.osuosl.org,
	jeffrey.t.kirsher@...el.com, jesse.brandeburg@...el.com,
	john.ronciak@...el.com, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, matthew.vick@...el.com,
	mitch.a.williams@...el.com, netdev@...r.kernel.org,
	shannon.nelson@...el.com
Subject: Re: [RFC Patch 08/12] IXGBEVF: Rework code of finding the end
 transmit desc of package

On Thu, Oct 22, 2015 at 12:37:40AM +0800, Lan Tianyu wrote:
> When transmit a package, the end transmit desc of package
> indicates whether package is sent already. Current code records
> the end desc's pointer in the next_to_watch of struct tx buffer.
> This code will be broken if shifting desc ring after migration.
> The pointer will be invalid. This patch is to replace recording
> pointer with recording the desc number of the package and find
> the end decs via the first desc and desc number.
> 
> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>

Do you really need to play the shifting games?
Can't you just reset everything and re-initialize the rings?
It's slower but way less intrusive.
Also removes the need to track writes into rings.

> ---
>  drivers/net/ethernet/intel/ixgbevf/ixgbevf.h      |  1 +
>  drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c | 19 ++++++++++++++++---
>  2 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h b/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
> index 775d089..c823616 100644
> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf.h
> @@ -54,6 +54,7 @@
>   */
>  struct ixgbevf_tx_buffer {
>  	union ixgbe_adv_tx_desc *next_to_watch;
> +	u16 desc_num;
>  	unsigned long time_stamp;
>  	struct sk_buff *skb;
>  	unsigned int bytecount;
> diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> index 4446916..056841c 100644
> --- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> +++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
> @@ -210,6 +210,7 @@ static void ixgbevf_unmap_and_free_tx_resource(struct ixgbevf_ring *tx_ring,
>  			       DMA_TO_DEVICE);
>  	}
>  	tx_buffer->next_to_watch = NULL;
> +	tx_buffer->desc_num = 0;
>  	tx_buffer->skb = NULL;
>  	dma_unmap_len_set(tx_buffer, len, 0);
>  	/* tx_buffer must be completely set up in the transmit path */
> @@ -295,7 +296,7 @@ static bool ixgbevf_clean_tx_irq(struct ixgbevf_q_vector *q_vector,
>  	union ixgbe_adv_tx_desc *tx_desc;
>  	unsigned int total_bytes = 0, total_packets = 0;
>  	unsigned int budget = tx_ring->count / 2;
> -	unsigned int i = tx_ring->next_to_clean;
> +	int i, watch_index;
>  
>  	if (test_bit(__IXGBEVF_DOWN, &adapter->state))
>  		return true;
> @@ -305,9 +306,17 @@ static bool ixgbevf_clean_tx_irq(struct ixgbevf_q_vector *q_vector,
>  	i -= tx_ring->count;
>  
>  	do {
> -		union ixgbe_adv_tx_desc *eop_desc = tx_buffer->next_to_watch;
> +		union ixgbe_adv_tx_desc *eop_desc;
> +
> +		if (!tx_buffer->desc_num)
> +			break;
> +
> +		if (i + tx_buffer->desc_num >= 0)
> +			watch_index = i + tx_buffer->desc_num;
> +		else
> +			watch_index = i + tx_ring->count + tx_buffer->desc_num;
>  
> -		/* if next_to_watch is not set then there is no work pending */
> +		eop_desc = IXGBEVF_TX_DESC(tx_ring, watch_index);
>  		if (!eop_desc)
>  			break;
>  
> @@ -320,6 +329,7 @@ static bool ixgbevf_clean_tx_irq(struct ixgbevf_q_vector *q_vector,
>  
>  		/* clear next_to_watch to prevent false hangs */
>  		tx_buffer->next_to_watch = NULL;
> +		tx_buffer->desc_num = 0;
>  
>  		/* update the statistics for this packet */
>  		total_bytes += tx_buffer->bytecount;
> @@ -3457,6 +3467,7 @@ static void ixgbevf_tx_map(struct ixgbevf_ring *tx_ring,
>  	u32 tx_flags = first->tx_flags;
>  	__le32 cmd_type;
>  	u16 i = tx_ring->next_to_use;
> +	u16 start;
>  
>  	tx_desc = IXGBEVF_TX_DESC(tx_ring, i);
>  
> @@ -3540,6 +3551,8 @@ static void ixgbevf_tx_map(struct ixgbevf_ring *tx_ring,
>  
>  	/* set next_to_watch value indicating a packet is present */
>  	first->next_to_watch = tx_desc;
> +	start = first - tx_ring->tx_buffer_info;
> +	first->desc_num = (i - start >= 0) ? i - start: i + tx_ring->count - start;
>  
>  	i++;
>  	if (i == tx_ring->count)
> -- 
> 1.8.4.rc0.1.g8f6a3e5.dirty
> 
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