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@xxxxxxxxxx> --- Documentation/virt/kvm/api.rst | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index abd7c32126ce..0912db16425f 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -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_ORDERED +---------------------------------------------------------- :Architectures: x86 :Parameters: args[0] - size of the dirty log ring @@ -8076,7 +8076,10 @@ The userspace should harvest this GFN and mark the flags from state to show that this GFN is harvested and waiting for a reset), and move 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. +all the dirty GFNs that were available. Note that on relaxed memory +architectures, userspace stores to the ring buffer must be ordered, +for example by using a store-release when available, or by using 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 @@ -8106,6 +8109,13 @@ 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_ORDERED is the only capability that can +be exposed by relaxed memory architecture, in order to indicate the +additional memory ordering requirements imposed on userspace when +mutating an entry from DIRTY to HARVESTED states. Architecture with +TSO-like ordering (such as x86) can expose both KVM_CAP_DIRTY_LOG_RING +and KVM_CAP_DIRTY_LOG_RING_ORDERED to userspace. + 8.30 KVM_CAP_XEN_HVM -------------------- -- 2.34.1
