On Fri,  2 Aug 2019 15:50:09 +0100
Steven Price [off-list ref] wrote:

[+Peter for the userspace aspect of things]

Hi Steve,

nit: AArch64

+
+https://developer.arm.com/docs/den0057/a

Between this file and the above document, which one is authoritative?

+
+KVM/Arm64 implements the stolen time part of this specification by providing

nit: KVM/arm64

+some hypervisor service calls to support a paravirtualized guest obtaining a
+view of the amount of time stolen from its execution.
+
+Two new SMCCC compatible hypercalls are defined:
+
+PV_FEATURES 0xC5000020
+PV_TIME_ST  0xC5000022
+
+These are only available in the SMC64/HVC64 calling convention as
+paravirtualized time is not available to 32 bit Arm guests.
+
+PV_FEATURES
+    Function ID:  (uint32)  : 0xC5000020
+    PV_func_id:   (uint32)  : Either PV_TIME_LPT or PV_TIME_ST
+    Return value: (int32)   : NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
+                              PV-time feature is supported by the hypervisor.

How is PV_FEATURES discovered? Is the intention to make it a generic
ARM-wide PV discovery mechanism, not specific to PV time?

+
+PV_TIME_ST
+    Function ID:  (uint32)  : 0xC5000022
+    Return value: (int64)   : IPA of the stolen time data structure for this
+                              (V)CPU. On failure:
+                              NOT_SUPPORTED (-1)
+

Is the size implicit? What are the memory attributes? This either needs
documenting here, or point to the right bit to the spec.

+Stolen Time
+-----------
+
+The structure pointed to by the PV_TIME_ST hypercall is as follows:
+
+  Field       | Byte Length | Byte Offset | Description
+  ----------- | ----------- | ----------- | --------------------------
+  Revision    |      4      |      0      | Must be 0 for version 0.1
+  Attributes  |      4      |      4      | Must be 0
+  Stolen time |      8      |      8      | Stolen time in unsigned
+              |             |             | nanoseconds indicating how
+              |             |             | much time this VCPU thread
+              |             |             | was involuntarily not
+              |             |             | running on a physical CPU.
+
+The structure will be updated by the hypervisor periodically as time is stolen

Is it really periodic? If so, when is the update frequency?

+from the VCPU. It will be present within a reserved region of the normal
+memory given to the guest. The guest should not attempt to write into this
+memory. There is a structure by VCPU of the guest.

What if the vcpu writes to it? Does it get a fault? If there is a
structure per vcpu, what is the layout in memory? How does a vcpu find
its own data structure? Is that the address returned by PV_TIME_ST?

+
+User space interface
+====================
+
+User space can request that KVM provide the paravirtualized time interface to
+a guest by creating a KVM_DEV_TYPE_ARM_PV_TIME device, for example:
+
+    struct kvm_create_device pvtime_device = {
+            .type = KVM_DEV_TYPE_ARM_PV_TIME,
+            .attr = 0,
+            .flags = 0,
+    };
+
+    pvtime_fd = ioctl(vm_fd, KVM_CREATE_DEVICE, &pvtime_device);
+
+The guest IPA of the structures must be given to KVM. This is the base address

nit: s/guest //

+of an array of stolen time structures (one for each VCPU). For example:
+
+    struct kvm_device_attr st_base = {
+            .group = KVM_DEV_ARM_PV_TIME_PADDR,
+            .attr = KVM_DEV_ARM_PV_TIME_ST,
+            .addr = (u64)(unsigned long)&st_paddr
+    };
+
+    ioctl(pvtime_fd, KVM_SET_DEVICE_ATTR, &st_base);

So the allocation itself is performed by the kernel? What are the
ordering requirements between creating vcpus and the device? What are
the alignment requirements for the base address?

+
+For migration (or save/restore) of a guest it is necessary to save the contents
+of the shared page(s) and later restore them. KVM_DEV_ARM_PV_TIME_STATE_SIZE
+provides the size of this data and KVM_DEV_ARM_PV_TIME_STATE allows the state
+to be read/written.

Is the size variable depending on the number of vcpus?

+
+It is also necessary for the physical address to be set identically when
+restoring.
+
+    void *save_state(int fd, u64 attr, u32 *size) {
+        struct kvm_device_attr get_size = {
+                .group = KVM_DEV_ARM_PV_TIME_STATE_SIZE,
+                .attr = attr,
+                .addr = (u64)(unsigned long)size
+        };
+
+        ioctl(fd, KVM_GET_DEVICE_ATTR, get_size);
+
+        void *buffer = malloc(*size);
+
+        struct kvm_device_attr get_state = {
+                .group = KVM_DEV_ARM_PV_TIME_STATE,
+                .attr = attr,
+                .addr = (u64)(unsigned long)size
+        };
+
+        ioctl(fd, KVM_GET_DEVICE_ATTR, buffer);
+    }
+
+    void *st_state = save_state(pvtime_fd, KVM_DEV_ARM_PV_TIME_ST, &st_size);
+

Thanks,

	M.
-- 
Without deviation from the norm, progress is not possible.

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