Date: Tue, 3 Mar 2020 10:11:28 +0100 From: Willem Jan Withagen <wjw@digiware.nl> To: cem@freebsd.org Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: What is the exact function of the offset parameter in preadv?? Message-ID: <896f3360-f4df-391a-3b08-47fd8de6eabb@digiware.nl> In-Reply-To: <CAG6CVpUAdOArvMWSDk5BOBsW0j3EYKKrxx0N0gYWNaacS87BKg@mail.gmail.com> References: <ecdc6007-e6dc-a30e-97fc-e18634d93430@digiware.nl> <CAG6CVpUAdOArvMWSDk5BOBsW0j3EYKKrxx0N0gYWNaacS87BKg@mail.gmail.com>
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On 3-3-2020 01:35, Conrad Meyer wrote:
> 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.
Conrad,
Right, I have annotated the code that does block_local writes, and I'm
in the
process of adding block_rbd to it. And it is from those that I get these
values.
So they are actual accesses to regular files.
But I think I got caught in 32->64 sign extention while printing the values.
And we are actually accessing things like `size(file) - 1024` .
I'll see if I can get a bit of extra doc in the manpage.
Thanx,
--WjW
> 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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