forked from Minki/linux
s390/docs: Fix the documentation of the address spaces
The information about the address spaces was completely outdated, since
the usage of the address spaces changed quite a bit since the early days.
This patch now updates the information about the usage of the address
spaces, mostly by using the description from Heiko's patch "rework uaccess
code - fix locking issues" (457f218095).
Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
This commit is contained in:
Thomas Huth
Martin Schwidefsky
parent
eaf785d51d
commit
b195562311
@ -114,28 +114,25 @@ s/390 z/Architecture
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16-17 16-17 Address Space Control
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00 Primary Space Mode when DAT on
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The linux kernel currently runs in this mode, CR1 is affiliated with
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this mode & points to the primary segment table origin etc.
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00 Primary Space Mode:
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The register CR1 contains the primary address-space control ele-
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ment (PASCE), which points to the primary space region/segment
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table origin.
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01 Access register mode this mode is used in functions to
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copy data between kernel & user space.
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01 Access register mode
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10 Secondary space mode not used in linux however CR7 the
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register affiliated with this mode is & this & normally
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CR13=CR7 to allow us to copy data between kernel & user space.
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We do this as follows:
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We set ar2 to 0 to designate its
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affiliated gpr ( gpr2 )to point to primary=kernel space.
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We set ar4 to 1 to designate its
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affiliated gpr ( gpr4 ) to point to secondary=home=user space
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& then essentially do a memcopy(gpr2,gpr4,size) to
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copy data between the address spaces, the reason we use home space for the
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kernel & don't keep secondary space free is that code will not run in
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secondary space.
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10 Secondary Space Mode:
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The register CR7 contains the secondary address-space control
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element (SASCE), which points to the secondary space region or
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segment table origin.
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11 Home Space Mode all user programs run in this mode.
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it is affiliated with CR13.
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11 Home Space Mode:
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The register CR13 contains the home space address-space control
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element (HASCE), which points to the home space region/segment
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table origin.
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See "Address Spaces on Linux for s/390 & z/Architecture" below
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for more information about address space usage in Linux.
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18-19 18-19 Condition codes (CC)
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@ -249,9 +246,9 @@ currently 4TB of physical memory currently on z/Architecture.
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Address Spaces on Linux for s/390 & z/Architecture
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==================================================
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Our addressing scheme is as follows
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Our addressing scheme is basically as follows:
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Primary Space Home Space
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Himem 0x7fffffff 2GB on s/390 ***************** ****************
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currently 0x3ffffffffff (2^42)-1 * User Stack * * *
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on z/Architecture. ***************** * *
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@ -264,9 +261,46 @@ on z/Architecture. ***************** * *
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* Sections * * *
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0x00000000 ***************** ****************
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This also means that we need to look at the PSW problem state bit
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or the addressing mode to decide whether we are looking at
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user or kernel space.
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This also means that we need to look at the PSW problem state bit and the
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addressing mode to decide whether we are looking at user or kernel space.
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User space runs in primary address mode (or access register mode within
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the vdso code).
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The kernel usually also runs in home space mode, however when accessing
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user space the kernel switches to primary or secondary address mode if
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the mvcos instruction is not available or if a compare-and-swap (futex)
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instruction on a user space address is performed.
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When also looking at the ASCE control registers, this means:
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User space:
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- runs in primary or access register mode
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- cr1 contains the user asce
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- cr7 contains the user asce
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- cr13 contains the kernel asce
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Kernel space:
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- runs in home space mode
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- cr1 contains the user or kernel asce
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-> the kernel asce is loaded when a uaccess requires primary or
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secondary address mode
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- cr7 contains the user or kernel asce, (changed with set_fs())
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- cr13 contains the kernel asce
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In case of uaccess the kernel changes to:
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- primary space mode in case of a uaccess (copy_to_user) and uses
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e.g. the mvcp instruction to access user space. However the kernel
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will stay in home space mode if the mvcos instruction is available
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- secondary space mode in case of futex atomic operations, so that the
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instructions come from primary address space and data from secondary
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space
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In case of KVM, the kernel runs in home space mode, but cr1 gets switched
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to contain the gmap asce before the SIE instruction gets executed. When
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the SIE instruction is finished, cr1 will be switched back to contain the
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user asce.
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Virtual Addresses on s/390 & z/Architecture
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===========================================
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