[PATCH 1/3] docs: Briefly describe KVM PV features

[Vitaly Kuznetsov <vkuznets@redhat.com>]

KVM PV features don't seem to be documented anywhere, in particular, the
fact that some of the features are enabled by default and some are not can
only be figured out from the code.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 docs/kvm-pv.txt | 92 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100644 docs/kvm-pv.txt

diff --git a/docs/kvm-pv.txt b/docs/kvm-pv.txt
new file mode 100644
index 000000000000..84ad7fa60f8d
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+KVM paravirtualized features
+============================
+
+
+1. Description
+===============
+In some cases when implementing a hardware interface in software is slow, KVM
+implements its own paravirtualized interfaces.
+
+2. Setup
+=========
+KVM PV features are represented as CPU flags. The following features are enabled
+by default for any CPU model when KVM is enabled:
+  kvmclock
+  kvm-nopiodelay
+  kvm-asyncpf
+  kvm-steal-time
+  kvm-pv-eoi
+  kvmclock-stable-bit
+
+'kvm-msi-ext-dest-id' feature is enabled by default in x2apic mode with split
+irqchip (e.g. "-machine ...,kernel-irqchip=split -cpu ...,x2apic").
+
+Note: when cpu model 'host' is used, QEMU passes through all KVM PV features
+exposed by KVM to the guest.
+
+3. Existing features
+====================
+
+3.1. kvmclock
+================
+This feature exposes KVM specific PV clocksource to the guest.
+
+3.2. kvm-nopiodelay
+===================
+The guest doesn't need to perform delays on PIO operations.
+
+3.3. kvm-mmu
+============
+This feature is deprecated.
+
+3.4. kvm-asyncpf
+================
+Enables asynchronous page fault mechanism. Note: since Linux-5.10 the feature is
+deprecated and not enabled by KVM. Use "kvm-asyncpf-int" instead.
+
+3.5. kvm-steal-time
+===================
+Enables stolen (when guest vCPU is not running) time accounting.
+
+3.6. kvm-pv-eoi
+===============
+Enables paravirtualized end-of-interrupt signaling.
+
+3.7. kvm-pv-unhalt
+==================
+Enables paravirtualized spinlocks support.
+
+3.8. kvm-pv-tlb-flush
+=====================
+Enables paravirtualized TLB flush mechanism.
+
+3.9. kvm-pv-ipi
+===============
+Enables paravirtualized IPI mechanism.
+
+3.10. kvm-poll-control
+======================
+Enables host-side polling on HLT control from the guest.
+
+3.11. kvm-pv-sched-yield
+========================
+Enables paravirtualized sched yield feature.
+
+3.12. kvm-asyncpf-int
+=====================
+Enables interrupt based asynchronous page fault mechanism.
+
+3.13. kvm-msi-ext-dest-id
+=========================
+Support 'Extended Destination ID' for external interrupts. The feature allows
+to use up to 32768 CPUs without IRQ remapping (but other limits may apply making
+the number of supported vCPUs for a given configuration lower).
+
+3.14. kvmclock-stable-bit
+=========================
+Tells the guest that guest visible TSC value can be fully trusted for kvmclock
+computations and no warps are expected.
+
+4. Useful links
+================
+Please refer to Documentation/virt/kvm in Linux for additional detail.
-- 
2.31.1