Date: Mon, 2 Mar 2020 16:35:25 -0800 From: Conrad Meyer <cem@freebsd.org> To: Willem Jan Withagen <wjw@digiware.nl> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: What is the exact function of the offset parameter in preadv?? Message-ID: <CAG6CVpUAdOArvMWSDk5BOBsW0j3EYKKrxx0N0gYWNaacS87BKg@mail.gmail.com> In-Reply-To: <ecdc6007-e6dc-a30e-97fc-e18634d93430@digiware.nl> References: <ecdc6007-e6dc-a30e-97fc-e18634d93430@digiware.nl>
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Hi Willem,
The offset parameter to the p{read,write}{,v} family of functions
indeed indicates a file offset. For ordinary files, negative offsets
are invalid (EINVAL).
Bhyve is probably accessing a special kernel device file, kmem(4), and
those negative "offsets" instead represent addresses in kernel virtual
memory.
Best,
Conrad
On Sun, Mar 1, 2020 at 4:47 PM Willem Jan Withagen <wjw@digiware.nl> wrote:
>
> Hi,
>
> Tried looking in the manual page, but it does not really describes what
> off_t offset does with a preadv...
>
> I suspect that is the offset in the file to start reading from?
>
> Next question is: What if the value is negative?
> Are we then reading from the end of the file/device if it is
> seekable?
>
> Manual page says:
> The pread() and preadv() system calls may also return the following
> errors:
> [EINVAL] The offset value was negative.
>
> But then I have this behyve code that gets calls like:
> block_proc_read: buf=0x0, cnt=1, len=512, off=-1024(0xfffffc00).
> block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00).
> block_proc_read: buf=0x0, cnt=1, len=512, off=-1024(0xfffffc00).
> block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00).
> block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00).
> block_proc_read: buf=0x0, cnt=1, len=2048, off=-32256(0xffff8200).
> block_proc_read: buf=0x0, cnt=1, len=512, off=512(0x200).
> block_proc_read: buf=0x0, cnt=1, len=512, off=0(0x0).
> block_proc_read: buf=0x0, cnt=1, len=512, off=512(0x200).
> block_proc_read: buf=0x0, cnt=1, len=512, off=0(0x0).
> block_proc_read: buf=0x0, cnt=1, len=512, off=512(0x200).
> block_proc_read: buf=0x0, cnt=1, len=16384, off=1024(0x400).
> block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00).
> block_proc_read: buf=0x0, cnt=1, len=16384, off=-16896(0xffffbe00).
> block_proc_read: buf=0x0, cnt=1, len=512, off=-512(0xfffffe00).
>
> Where these are the very first calls that a bhyve loaded image executes.
> which I guess is for probing the bootdisk to see where/what it needs to
> mount a root disk.
>
> Below bhyve boot output of a not working boot, and a working boot.
> The read trace above is from the working bhyve boot.
>
> So what are the negative offsets?
>
> --WjW
>
>
> Compare
> Using block_rbd_read for reading from a Rados device
>
> =============
> vtblk0: <VirtIO Block Adapter> on virtio_pci1
> vtblk0: 134217727MB (34359738366 4096 byte sectors)
> ahci0: <Intel ICH8 AHCI SATA controller> mem 0xc0004000-0xc00043ff irq
> 18 at device 4.0 on pci0
> ahci0: AHCI v1.30 with 6 6Gbps ports, Port Multiplier not supported
> atkbdc0: <Keyboard controller (i8042)> port 0x60,0x64 irq 1 on acpi0
> atkbd0: <AT Keyboard> irq 1 on atkbdc0
> kbd0 at atkbd0
> atkbd0: [GIANT-LOCKED]
> driver bug: Unable to set devclass (class: atkbdc devname: (unknown))
> psm0: <PS/2 Mouse> irq 12 on atkbdc0
> psm0: [GIANT-LOCKED]
> psm0: model Generic PS/2 mouse, device ID 0
> uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
> uart0: console (9600,n,8,1)
> uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0
> vga0: <Generic ISA VGA> at port 0x3b0-0x3bb iomem 0xb0000-0xb7fff pnpid
> PNP0900 on isa0
> Timecounters tick every 10.000 msec
> usb_needs_explore_all: no devclass
> Trying to mount root from ufs:/dev/ada0p2 [rw]...
> mountroot: waiting for device /dev/ada0p2...
> Mounting from ufs:/dev/ada0p2 failed with error 19.
>
> Loader variables:
> vfs.root.mountfrom=ufs:/dev/ada0p2
> vfs.root.mountfrom.options=rw
>
> Manual root filesystem specification:
> <fstype>:<device> [options]
> Mount <device> using filesystem <fstype>
> and with the specified (optional) option list.
>
> eg. ufs:/dev/da0s1a
> zfs:zroot/ROOT/default
> cd9660:/dev/cd0 ro
> (which is equivalent to: mount -t cd9660 -o ro /dev/cd0 /)
>
> ? List valid disk boot devices
> . Yield 1 second (for background tasks)
> <empty line> Abort manual input
>
> =============
>
> With booting from a normal file device:
> atkbdc0: <Keyboard controller (i8042)> port 0x60,0x64 irq 1 on acpi0
> atkbd0: <AT Keyboard> irq 1 on atkbdc0
> kbd0 at atkbd0
> atkbd0: [GIANT-LOCKED]
> driver bug: Unable to set devclass (class: atkbdc devname: (unknown))
> psm0: <PS/2 Mouse> irq 12 on atkbdc0
> psm0: [GIANT-LOCKED]
> psm0: model Generic PS/2 mouse, device ID 0
> uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
> uart0: console (9600,n,8,1)
> uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0
> vga0: <Generic ISA VGA> at port 0x3b0-0x3bb iomem 0xb0000-0xb7fff pnpid
> PNP0900 on isa0
> Timecounters tick every 10.000 msec
> usb_needs_explore_all: no devclass
> Trying to mount root from ufs:/dev/ada0p2 [rw]...
> Root mount waiting for: CAM
> ada0 at ahcich0 bus 0 scbus0 target 0 lun 0
> ada0: <BHYVE SATA DISK 001> ACS-2 ATA SATA 3.x device
> ada0: Serial Number BHYVE-8C7D-D922-AD47
> ada0: 600.000MB/s transfers (SATA 3.x, UDMA6, PIO 8192bytes)
> ada0: Command Queueing enabled
> ada0: 20480MB (41943040 512 byte sectors)
> ada1 at ahcich6 bus 0 scbus1 target 0 lun 0
> ada1: <BHYVE SATA DISK 001> ACS-2 ATA SATA 3.x device
> ada1: Serial Number BHYVE-4318-7904-5D51
> ada1: 600.000MB/s transfers (SATA 3.x, UDMA6, PIO 8192bytes)
> ada1: Command Queueing enabled
> ada1: 20480MB (41943040 512 byte sectors)
> mountroot: waiting for device /dev/ada0p2...
> WARNING: / was not properly dismounted
> WARNING: /: mount pending error: blocks 168 files 1
> ==========
>
>
>
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