diff options
Diffstat (limited to 'Documentation/virtual/kvm')
| -rw-r--r-- | Documentation/virtual/kvm/api.txt | 17 | ||||
| -rw-r--r-- | Documentation/virtual/kvm/halt-polling.txt | 37 | ||||
| -rw-r--r-- | Documentation/virtual/kvm/mmu.txt | 41 |
3 files changed, 49 insertions, 46 deletions
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt index 356156f5c52d..7de9eee73fcd 100644 --- a/Documentation/virtual/kvm/api.txt +++ b/Documentation/virtual/kvm/api.txt @@ -45,6 +45,23 @@ the API. The only supported use is one virtual machine per process, and one vcpu per thread. +It is important to note that althought VM ioctls may only be issued from +the process that created the VM, a VM's lifecycle is associated with its +file descriptor, not its creator (process). In other words, the VM and +its resources, *including the associated address space*, are not freed +until the last reference to the VM's file descriptor has been released. +For example, if fork() is issued after ioctl(KVM_CREATE_VM), the VM will +not be freed until both the parent (original) process and its child have +put their references to the VM's file descriptor. + +Because a VM's resources are not freed until the last reference to its +file descriptor is released, creating additional references to a VM via +via fork(), dup(), etc... without careful consideration is strongly +discouraged and may have unwanted side effects, e.g. memory allocated +by and on behalf of the VM's process may not be freed/unaccounted when +the VM is shut down. + + 3. Extensions ------------- diff --git a/Documentation/virtual/kvm/halt-polling.txt b/Documentation/virtual/kvm/halt-polling.txt index 4a8418318769..4f791b128dd2 100644 --- a/Documentation/virtual/kvm/halt-polling.txt +++ b/Documentation/virtual/kvm/halt-polling.txt @@ -53,7 +53,8 @@ the global max polling interval then the polling interval can be increased in the hope that next time during the longer polling interval the wake up source will be received while the host is polling and the latency benefits will be received. The polling interval is grown in the function grow_halt_poll_ns() and -is multiplied by the module parameter halt_poll_ns_grow. +is multiplied by the module parameters halt_poll_ns_grow and +halt_poll_ns_grow_start. In the event that the total block time was greater than the global max polling interval then the host will never poll for long enough (limited by the global @@ -80,22 +81,30 @@ shrunk. These variables are defined in include/linux/kvm_host.h and as module parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the powerpc kvm-hv case. -Module Parameter | Description | Default Value +Module Parameter | Description | Default Value -------------------------------------------------------------------------------- -halt_poll_ns | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT - | which defines the ceiling value | - | of the polling interval for | (per arch value) - | each vcpu. | +halt_poll_ns | The global max polling | KVM_HALT_POLL_NS_DEFAULT + | interval which defines | + | the ceiling value of the | + | polling interval for | (per arch value) + | each vcpu. | -------------------------------------------------------------------------------- -halt_poll_ns_grow | The value by which the halt | 2 - | polling interval is multiplied | - | in the grow_halt_poll_ns() | - | function. | +halt_poll_ns_grow | The value by which the | 2 + | halt polling interval is | + | multiplied in the | + | grow_halt_poll_ns() | + | function. | -------------------------------------------------------------------------------- -halt_poll_ns_shrink | The value by which the halt | 0 - | polling interval is divided in | - | the shrink_halt_poll_ns() | - | function. | +halt_poll_ns_grow_start | The initial value to grow | 10000 + | to from zero in the | + | grow_halt_poll_ns() | + | function. | +-------------------------------------------------------------------------------- +halt_poll_ns_shrink | The value by which the | 0 + | halt polling interval is | + | divided in the | + | shrink_halt_poll_ns() | + | function. | -------------------------------------------------------------------------------- These module parameters can be set from the debugfs files in: diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt index e507a9e0421e..f365102c80f5 100644 --- a/Documentation/virtual/kvm/mmu.txt +++ b/Documentation/virtual/kvm/mmu.txt @@ -224,10 +224,6 @@ Shadow pages contain the following information: A bitmap indicating which sptes in spt point (directly or indirectly) at pages that may be unsynchronized. Used to quickly locate all unsychronized pages reachable from a given page. - mmu_valid_gen: - Generation number of the page. It is compared with kvm->arch.mmu_valid_gen - during hash table lookup, and used to skip invalidated shadow pages (see - "Zapping all pages" below.) clear_spte_count: Only present on 32-bit hosts, where a 64-bit spte cannot be written atomically. The reader uses this while running out of the MMU lock @@ -402,27 +398,6 @@ causes its disallow_lpage to be incremented, thus preventing instantiation of a large spte. The frames at the end of an unaligned memory slot have artificially inflated ->disallow_lpages so they can never be instantiated. -Zapping all pages (page generation count) -========================================= - -For the large memory guests, walking and zapping all pages is really slow -(because there are a lot of pages), and also blocks memory accesses of -all VCPUs because it needs to hold the MMU lock. - -To make it be more scalable, kvm maintains a global generation number -which is stored in kvm->arch.mmu_valid_gen. Every shadow page stores -the current global generation-number into sp->mmu_valid_gen when it -is created. Pages with a mismatching generation number are "obsolete". - -When KVM need zap all shadow pages sptes, it just simply increases the global -generation-number then reload root shadow pages on all vcpus. As the VCPUs -create new shadow page tables, the old pages are not used because of the -mismatching generation number. - -KVM then walks through all pages and zaps obsolete pages. While the zap -operation needs to take the MMU lock, the lock can be released periodically -so that the VCPUs can make progress. - Fast invalidation of MMIO sptes =============================== @@ -435,8 +410,7 @@ shadow pages, and is made more scalable with a similar technique. MMIO sptes have a few spare bits, which are used to store a generation number. The global generation number is stored in kvm_memslots(kvm)->generation, and increased whenever guest memory info -changes. This generation number is distinct from the one described in -the previous section. +changes. When KVM finds an MMIO spte, it checks the generation number of the spte. If the generation number of the spte does not equal the global generation @@ -452,13 +426,16 @@ stored into the MMIO spte. Thus, the MMIO spte might be created based on out-of-date information, but with an up-to-date generation number. To avoid this, the generation number is incremented again after synchronize_srcu -returns; thus, the low bit of kvm_memslots(kvm)->generation is only 1 during a +returns; thus, bit 63 of kvm_memslots(kvm)->generation set to 1 only during a memslot update, while some SRCU readers might be using the old copy. We do not want to use an MMIO sptes created with an odd generation number, and we can do -this without losing a bit in the MMIO spte. The low bit of the generation -is not stored in MMIO spte, and presumed zero when it is extracted out of the -spte. If KVM is unlucky and creates an MMIO spte while the low bit is 1, -the next access to the spte will always be a cache miss. +this without losing a bit in the MMIO spte. The "update in-progress" bit of the +generation is not stored in MMIO spte, and is so is implicitly zero when the +generation is extracted out of the spte. If KVM is unlucky and creates an MMIO +spte while an update is in-progress, the next access to the spte will always be +a cache miss. For example, a subsequent access during the update window will +miss due to the in-progress flag diverging, while an access after the update +window closes will have a higher generation number (as compared to the spte). Further reading |