Hi all,
In case of SCSI passthrough, If the Block Limits VPD device response is
absent from hardware, QEMU handles it.
There are several variables involved in this process as follows:
* The bl.max_transfer
* The bl.max_iov that is associated with IOV_MAX.
* The bl.max_hw_iov that is associated with the max_segments sysfs
setting for the relevant block device on the host.
* The bl.max_hw_transfer that is associated with the BLKSECTGET ioctl,
in other words related to the current max_sectors_kb sysfs setting of
the relevant block device on the host.
Then take the smallest value and return it as the result of "Maximum
transfer length" after relevant calculation, See:
static uint64_t calculate_max_transfer(SCSIDevice *s)
{
uint64_t max_transfer = blk_get_max_hw_transfer(s->conf.blk);
uint32_t max_iov = blk_get_max_hw_iov(s->conf.blk);
assert(max_transfer);
max_transfer = MIN_NON_ZERO(max_transfer,
max_iov * qemu_real_host_page_size());
return max_transfer / s->blocksize;
}
However, due to the limitation of IOV_MAX, no matter how powerful the
host scsi hardware is, the "Maximum transfer length" that qemu emulates
in bl vpd page is capped at 8192 sectors in case of 4kb page size and
512 bytes logical block size.
For example：
host:~ # sg_vpd -p bl /dev/sda
Block limits VPD page (SBC)
......
Maximum transfer length: 0 blocks [not reported]
......
host:~ # cat /sys/class/block/sda/queue/max_sectors_kb
16384
host:~ # cat /sys/class/block/sda/queue/max_hw_sectors_kb
32767
host:~ # cat /sys/class/block/sda/queue/max_segments
4096
Expected:
guest:~ # sg_vpd -p bl /dev/sda
Block limits VPD page (SBC)
......
Maximum transfer length: 0x8000
......
guest:~ # cat /sys/class/block/sda/queue/max_sectors_kb
16384
guest:~ # cat /sys/class/block/sda/queue/max_hw_sectors_kb
32767
Actual:
guest:~ # sg_vpd -p bl /dev/sda
Block limits VPD page (SBC)
......
Maximum transfer length: 0x2000
......
guest:~ # cat /sys/class/block/sda/queue/max_sectors_kb
4096
guest:~ # cat /sys/class/block/sda/queue/max_hw_sectors_kb
32767
It seems the current design logic is not able to fully utilize the
performance of the scsi hardware. I have two questions:
1. I'm curious that is it reasonable to drop the logic about IOV_MAX
limitation, directly use the return value of BLKSECTGET as the maximum
transfer length when QEMU emulates the block limit page of scsi vpd?
If we doing so, we will have maximum transfer length in the guest
that is consistent with the capabilities of the host hardware。
2. Besides, Assume I set a value(eg: 8192 in kb) to max_sectors_kb in
guest which doesn't exceed the capabilities of the host hardware(eg:
16384 in kb) but exceeds the limit(eg: 4096 in kb) caused by IOV_MAX,
Any risks in readv()/writev() of raw-posix?
Lin
