From: Andrea Parri (Microsoft) Sent: Thursday,
December 17, 2020 12:33 PM
>
> Current code overestimates the value of max_outstanding_req_per_channel
> for Win8 and newer hosts, since vmscsi_size_delta is set to the initial
> value of sizeof(vmscsi_win8_extension) rather than zero. This may lead
> to wrong decisions when using ring_avail_percent_lowater equals to zero.
> The estimate of max_outstanding_req_per_channel is 'exact' for Win7 and
> older hosts. A better choice, keeping the algorithm for the estimation
> simple, is to err the other way around, i.e., to underestimate for Win7
> and older but to use the exact value for Win8 and newer.
>
> Suggested-by: Dexuan Cui
> Signed-off-by: Andrea Parri (Microsoft)
> Cc: "James E.J. Bottomley"
> Cc: "Martin K. Petersen"
> Cc: linux-s...@vger.kernel.org
> ---
> drivers/scsi/storvsc_drv.c | 7 +--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index ded00a89bfc4e..64298aa2f151e 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -2141,12 +2141,15 @@ static int __init storvsc_drv_init(void)
>* than the ring buffer size since that page is reserved for
>* the ring buffer indices) by the max request size (which is
>* vmbus_channel_packet_multipage_buffer + struct vstor_packet + u64)
> + *
> + * The computation underestimates max_outstanding_req_per_channel
> + * for Win7 and older hosts because it does not take into account
> + * the vmscsi_size_delta correction to the max request size.
>*/
> max_outstanding_req_per_channel =
> ((storvsc_ringbuffer_size - PAGE_SIZE) /
> ALIGN(MAX_MULTIPAGE_BUFFER_PACKET +
> - sizeof(struct vstor_packet) + sizeof(u64) -
> - vmscsi_size_delta,
> + sizeof(struct vstor_packet) + sizeof(u64),
> sizeof(u64)));
>
> #if IS_ENABLED(CONFIG_SCSI_FC_ATTRS)
> --
> 2.25.1
Reviewed-by: Michael Kelley
