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KVM: Document weakly ordered architecture requirements for dirty ring

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Now that the kernel can expose to userspace that its dirty ring
management relies on explicit ordering, document these new requirements
for VMMs to do the right thing.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20220926145120.27974-5-maz@kernel.org
This commit is contained in:
Marc Zyngier 2022-09-26 15:51:18 +01:00
parent fc0693d4e5
commit 671c8c7f9f
1 changed files with 15 additions and 2 deletions
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17
Documentation/virt/kvm/api.rst
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@ -8019,8 +8019,8 @@ guest according to the bits in the KVM_CPUID_FEATURES CPUID leaf
(0x40000001). Otherwise, a guest may use the paravirtual features
regardless of what has actually been exposed through the CPUID leaf.
8.29 KVM_CAP_DIRTY_LOG_RING
---------------------------
8.29 KVM_CAP_DIRTY_LOG_RING/KVM_CAP_DIRTY_LOG_RING_ACQ_REL
----------------------------------------------------------
:Architectures: x86
:Parameters: args[0] - size of the dirty log ring
@ -8078,6 +8078,11 @@ on to the next GFN. The userspace should continue to do this until the
flags of a GFN have the DIRTY bit cleared, meaning that it has harvested
all the dirty GFNs that were available.
Note that on weakly ordered architectures, userspace accesses to the
ring buffer (and more specifically the 'flags' field) must be ordered,
using load-acquire/store-release accessors when available, or any
other memory barrier that will ensure this ordering.
It's not necessary for userspace to harvest the all dirty GFNs at once.
However it must collect the dirty GFNs in sequence, i.e., the userspace
program cannot skip one dirty GFN to collect the one next to it.
@ -8106,6 +8111,14 @@ KVM_CAP_DIRTY_LOG_RING with an acceptable dirty ring size, the virtual
machine will switch to ring-buffer dirty page tracking and further
KVM_GET_DIRTY_LOG or KVM_CLEAR_DIRTY_LOG ioctls will fail.
NOTE: KVM_CAP_DIRTY_LOG_RING_ACQ_REL is the only capability that
should be exposed by weakly ordered architecture, in order to indicate
the additional memory ordering requirements imposed on userspace when
reading the state of an entry and mutating it from DIRTY to HARVESTED.
Architecture with TSO-like ordering (such as x86) are allowed to
expose both KVM_CAP_DIRTY_LOG_RING and KVM_CAP_DIRTY_LOG_RING_ACQ_REL
to userspace.
8.30 KVM_CAP_XEN_HVM
--------------------