man/man9/sglist.9

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.Dd April 24, 2020
.Dt SGLIST 9
.Os
.Sh NAME
.Nm sglist ,
.Nm sglist_alloc ,
.Nm sglist_append ,
.Nm sglist_append_bio ,
.Nm sglist_append_mbuf ,
.Nm sglist_append_mbuf_epg,
.Nm sglist_append_phys ,
.Nm sglist_append_sglist ,
.Nm sglist_append_uio ,
.Nm sglist_append_user ,
.Nm sglist_append_vmpages ,
.Nm sglist_build ,
.Nm sglist_clone ,
.Nm sglist_consume_uio ,
.Nm sglist_count ,
.Nm sglist_count_mbuf_epg ,
.Nm sglist_count_vmpages ,
.Nm sglist_free ,
.Nm sglist_hold ,
.Nm sglist_init ,
.Nm sglist_join ,
.Nm sglist_length ,
.Nm sglist_reset ,
.Nm sglist_slice ,
.Nm sglist_split
.Nd manage a scatter/gather list of physical memory addresses
.Sh SYNOPSIS
.In sys/types.h
.In sys/sglist.h
.Ft struct sglist *
.Fn sglist_alloc "int nsegs" "int mflags"
.Ft int
.Fn sglist_append "struct sglist *sg" "void *buf" "size_t len"
.Ft int
.Fn sglist_append_bio "struct sglist *sg" "struct bio *bp"
.Ft int
.Fn sglist_append_mbuf_epg "struct sglist *sg" "struct mbuf *m" "size_t offset" "size_t len"
.Ft int
.Fn sglist_append_mbuf "struct sglist *sg" "struct mbuf *m"
.Ft int
.Fn sglist_append_phys "struct sglist *sg" "vm_paddr_t paddr" "size_t len"
.Ft int
.Fn sglist_append_sglist "struct sglist *sg" "struct sglist *source" "size_t offset" "size_t len"
.Ft int
.Fn sglist_append_uio "struct sglist *sg" "struct uio *uio"
.Ft int
.Fn sglist_append_user "struct sglist *sg" "void *buf" "size_t len" "struct thread *td"
.Ft int
.Fn sglist_append_vmpages "struct sglist *sg" "vm_page_t *m" "size_t pgoff" "size_t len"
.Ft struct sglist *
.Fn sglist_build "void *buf" "size_t len" "int mflags"
.Ft struct sglist *
.Fn sglist_clone "struct sglist *sg" "int mflags"
.Ft int
.Fn sglist_consume_uio "struct sglist *sg" "struct uio *uio" "size_t resid"
.Ft int
.Fn sglist_count "void *buf" "size_t len"
.Ft int
.Fn sglist_count_mbuf_epg "struct mbuf *m" "size_t offset" "size_t len"
.Ft int
.Fn sglist_count_vmpages "vm_page_t *m" "size_t pgoff" "size_t len"
.Ft void
.Fn sglist_free "struct sglist *sg"
.Ft struct sglist *
.Fn sglist_hold "struct sglist *sg"
.Ft void
.Fn sglist_init "struct sglist *sg" "int maxsegs" "struct sglist_seg *segs"
.Ft int
.Fn sglist_join "struct sglist *first" "struct sglist *second"
.Ft size_t
.Fn sglist_length "struct sglist *sg"
.Ft void
.Fn sglist_reset "struct sglist *sg"
.Ft int
.Fn sglist_slice "struct sglist *original" "struct sglist **slice" "size_t offset" "size_t length" "int mflags"
.Ft int
.Fn sglist_split "struct sglist *original" "struct sglist **head" "size_t length" "int mflags"
.Sh DESCRIPTION
The
.Nm
API manages physical address ranges.
Each list contains one or more elements.
Each element contains a starting physical address and a length.
Scatter/gather lists are read-only while they are shared.
If one wishes to alter an existing scatter/gather list and does not hold the
sole reference to the list,
then one should create a new list instead of modifying the existing list.
.Pp
Each scatter/gather list object contains a reference count.
New lists are created with a single reference.
New references are obtained by calling
.Nm sglist_hold
and are released by calling
.Nm sglist_free .
.Ss Allocating and Initializing Lists
Each
.Nm
object consists of a header structure and a variable-length array of
scatter/gather list elements.
The
.Nm sglist_alloc
function allocates a new list that contains a header and
.Fa nsegs
scatter/gather list elements.
The
.Fa mflags
argument can be set to either
.Dv M_NOWAIT
or
.Dv M_WAITOK .
.Pp
The
.Nm sglist_count
function returns the number of scatter/gather list elements needed to describe
the physical address ranges mapped by a single kernel virtual address range.
The kernel virtual address range starts at
.Fa buf
and is
.Fa len
bytes long.
.Pp
The
.Nm sglist_count_mbuf_epg
function returns the number of scatter/gather list elements needed to describe
the external multipage mbuf buffer
.Fa m .
The ranges start at an offset of
.Fa offset
relative to the start of the buffer and is
.Fa len
bytes long.
.Pp
The
.Nm sglist_count_vmpages
function returns the number of scatter/gather list elements needed to describe
the physical address ranges of a buffer backed by an array of virtual memory
pages
.Fa m .
The buffer starts at an offset of
.Fa pgoff
bytes relative to the first page and is
.Fa len
bytes long.
.Pp
The
.Nm sglist_build
function allocates a new scatter/gather list object that describes the physical
address ranges mapped by a single kernel virtual address range.
The kernel virtual address range starts at
.Fa buf
and is
.Fa len
bytes long.
The
.Fa mflags
argument can be set to either
.Dv M_NOWAIT
or
.Dv M_WAITOK .
.Pp
The
.Nm sglist_clone
function returns a copy of an existing scatter/gather list object
.Fa sg .
The
.Fa mflags
argument can be set to either
.Dv M_NOWAIT
or
.Dv M_WAITOK .
This can be used to obtain a private copy of a scatter/gather list before
modifying it.
.Pp
The
.Nm sglist_init
function initializes a scatter/gather list header.
The header is pointed to by
.Fa sg
and is initialized to manage an array of
.Fa maxsegs
scatter/gather list elements pointed to by
.Fa segs .
This can be used to initialize a scatter/gather list header whose storage
is not provided by
.Nm sglist_alloc .
In that case, the caller should not call
.Nm sglist_free
to release its own reference and is responsible for ensuring all other
references to the list are dropped before it releases the storage for
.Fa sg
and
.Fa segs .
.Ss Constructing Scatter/Gather Lists
The
.Nm
API provides several routines for building a scatter/gather list to describe
one or more objects.
Specifically, the
.Nm sglist_append
family of routines can be used to append the physical address ranges described
by an object to the end of a scatter/gather list.
All of these routines return 0 on success or an error on failure.
If a request to append an address range to a scatter/gather list fails,
the scatter/gather list will remain unchanged.
.Pp
The
.Nm sglist_append
function appends the physical address ranges described by a single kernel
virtual address range to the scatter/gather list
.Fa sg .
The kernel virtual address range starts at
.Fa buf
and is
.Fa len
bytes long.
.Pp
The
.Nm sglist_append_bio
function appends the physical address ranges described by a single bio
.Fa bp
to the scatter/gather list
.Fa sg .
.Pp
The
.Nm sglist_append_mbuf_epg
function appends the physical address ranges described by the
external multipage
.Xr mbuf 9
buffer
.Fa ext_pgs
to the scatter/gather list
.Fa sg .
The physical address ranges start at offset
.Fa offset
within
.Fa ext_pgs
and continue for
.Fa len
bytes.
Note that unlike
.Nm sglist_append_mbuf ,
.Nm sglist_append_mbuf_epg
only adds ranges for a single mbuf,
not an entire mbuf chain.
.Pp
The
.Nm sglist_append_mbuf
function appends the physical address ranges described by an entire mbuf
chain
.Fa m
to the scatter/gather list
.Fa sg .
.Pp
The
.Nm sglist_append_phys
function appends a single physical address range to the scatter/gather list
.Fa sg .
The physical address range starts at
.Fa paddr
and is
.Fa len
bytes long.
.Pp
The
.Nm sglist_append_sglist
function appends physical address ranges described by the scatter/gather list
.Fa source
to the scatter/gather list
.Fa sg .
The physical address ranges start at offset
.Fa offset
within
.Fa source
and continue for
.Fa len
bytes.
.Pp
The
.Nm sglist_append_uio
function appends the physical address ranges described by a
.Xr uio 9
object to the scatter/gather list
.Fa sg .
Note that it is the caller's responsibility to ensure that the pages backing
the I/O request are wired for the lifetime of
.Fa sg .
Note also that this routine does not modify
.Fa uio .
.Pp
The
.Nm sglist_append_user
function appends the physical address ranges described by a single user
virtual address range to the scatter/gather list
.Fa sg .
The user virtual address range is relative to the address space of the thread
.Fa td .
It starts at
.Fa buf
and is
.Fa len
bytes long.
Note that it is the caller's responsibility to ensure that the pages backing
the user buffer are wired for the lifetime of
.Fa sg .
.Pp
The
.Nm sglist_append_vmpages
function appends the physical address ranges of a buffer backed by an array
of virtual memory pages
.Fa m .
The buffer starts at an offset of
.Fa pgoff
bytes relative to the first page and is
.Fa len
bytes long.
.Pp
The
.Nm sglist_consume_uio
function is a variation of
.Nm sglist_append_uio .
As with
.Nm sglist_append_uio ,
it appends the physical address ranges described by
.Fa uio
to the scatter/gather list
.Fa sg .
Unlike
.Nm sglist_append_uio ,
however,
.Nm sglist_consume_uio
modifies the I/O request to indicate that the appended address ranges have
been processed similar to calling
.Xr uiomove 9 .
This routine will only append ranges that describe up to
.Fa resid
total bytes in length.
If the available segments in the scatter/gather list are exhausted before
.Fa resid
bytes are processed,
then the
.Fa uio
structure will be updated to reflect the actual number of bytes processed,
and
.Nm sglist_consume_io
will return zero to indicate success.
In effect, this function will perform partial reads or writes.
The caller can compare the
.Fa uio_resid
member of
.Fa uio
before and after calling
.Nm sglist_consume_uio
to determine the actual number of bytes processed.
.Ss Manipulating Scatter/Gather Lists
The
.Nm sglist_join
function appends physical address ranges from the scatter/gather list
.Fa second
onto
.Fa first
and then resets
.Fa second
to an empty list.
It returns zero on success or an error on failure.
.Pp
The
.Nm sglist_split
function splits an existing scatter/gather list into two lists.
The first
.Fa length
bytes described by the list
.Fa original
are moved to a new list
.Fa *head .
If
.Fa original
describes a total address range that is smaller than
.Fa length
bytes,
then all of the address ranges will be moved to the new list at
.Fa *head
and
.Fa original
will be an empty list.
The caller may supply an existing scatter/gather list in
.Fa *head .
If so, the list must be empty.
Otherwise, the caller may set
.Fa *head
to
.Dv NULL
in which case a new scatter/gather list will be allocated.
In that case,
.Fa mflags
may be set to either
.Dv M_NOWAIT
or
.Dv M_WAITOK .
Note that since the
.Fa original
list is modified by this call, it must be a private list with no other
references.
The
.Nm sglist_split
function returns zero on success or an error on failure.
.Pp
The
.Nm sglist_slice
function generates a new scatter/gather list from a sub-range of an existing
scatter/gather list
.Fa original .
The sub-range to extract is specified by the
.Fa offset
and
.Fa length
parameters.
The new scatter/gather list is stored in
.Fa *slice .
As with
.Fa head
for
.Nm sglist_join ,
the caller may either provide an empty scatter/gather list,
or it may set
.Fa *slice
to
.Dv NULL
in which case
.Nm sglist_slice
will allocate a new list subject to
.Fa mflags .
Unlike
.Nm sglist_split ,
.Nm sglist_slice
does not modify
.Fa original
and does not require it to be a private list.
The
.Nm sglist_split
function returns zero on success or an error on failure.
.Ss Miscellaneous Routines
The
.Nm sglist_reset
function clears the scatter/gather list
.Fa sg
so that it no longer maps any address ranges.
This can allow reuse of a single scatter/gather list object for multiple
requests.
.Pp
The
.Nm sglist_length
function returns the total length of the physical address ranges described
by the scatter/gather list
.Fa sg .
.Sh RETURN VALUES
The
.Nm sglist_alloc ,
.Nm sglist_build ,
and
.Nm sglist_clone
functions return a new scatter/gather list on success or
.Dv NULL
on failure.
.Pp
The
.Nm sglist_append
family of functions and the
.Nm sglist_consume_uio ,
.Nm sglist_join ,
.Nm sglist_slice ,
and
.Nm sglist_split
functions return zero on success or an error on failure.
.Pp
The
.Nm sglist_count
family of
functions return a count of scatter/gather list elements.
.Pp
The
.Nm sglist_length
function returns a count of address space described by a scatter/gather list
in bytes.
.Sh ERRORS
The
.Nm sglist_append
functions return the following errors on failure:
.Bl -tag -width Er
.It Bq Er EINVAL
The scatter/gather list has zero segments.
.It Bq Er EFBIG
There are not enough available segments in the scatter/gather list to append
the specified physical address ranges.
.El
.Pp
The
.Nm sglist_consume_uio
function returns the following error on failure:
.Bl -tag -width Er
.It Bq Er EINVAL
The scatter/gather list has zero segments.
.El
.Pp
The
.Nm sglist_join
function returns the following error on failure:
.Bl -tag -width Er
.It Bq Er EFBIG
There are not enough available segments in the scatter/gather list
.Fa first
to append the physical address ranges from
.Fa second .
.El
.Pp
The
.Nm sglist_slice
function returns the following errors on failure:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa original
scatter/gather list does not describe enough address space to cover the
requested sub-range.
.It Bq Er EINVAL
The caller-supplied scatter/gather list in
.Fa *slice
is not empty.
.It Bq Er ENOMEM
An attempt to allocate a new scatter/gather list with
.Dv M_NOWAIT
set in
.Fa mflags
failed.
.It Bq Er EFBIG
There are not enough available segments in the caller-supplied scatter/gather
list in
.Fa *slice
to describe the requested physical address ranges.
.El
.Pp
The
.Nm sglist_split
function returns the following errors on failure:
.Bl -tag -width Er
.It Bq Er EDOOFUS
The
.Fa original
scatter/gather list has more than one reference.
.It Bq Er EINVAL
The caller-supplied scatter/gather list in
.Fa *head
is not empty.
.It Bq Er ENOMEM
An attempt to allocate a new scatter/gather list with
.Dv M_NOWAIT
set in
.Fa mflags
failed.
.It Bq Er EFBIG
There are not enough available segments in the caller-supplied scatter/gather
list in
.Fa *head
to describe the requested physical address ranges.
.El
.Sh SEE ALSO
.Xr g_bio 9 ,
.Xr malloc 9 ,
.Xr mbuf 9 ,
.Xr uio 9
.Sh HISTORY
This API was first introduced in
.Fx 8.0 .