On 28/08/2019 14:49, Christoffer Dall wrote: > On Tue, Aug 27, 2019 at 10:57:06AM +0200, Christoffer Dall wrote: >> On Wed, Aug 21, 2019 at 04:36:47PM +0100, Steven Price wrote: >>> Introduce a paravirtualization interface for KVM/arm64 based on the >>> "Arm Paravirtualized Time for Arm-Base Systems" specification DEN 0057A. >>> >>> This only adds the details about "Stolen Time" as the details of "Live >>> Physical Time" have not been fully agreed. >>> >>> User space can specify a reserved area of memory for the guest and >>> inform KVM to populate the memory with information on time that the host >>> kernel has stolen from the guest. >>> >>> A hypercall interface is provided for the guest to interrogate the >>> hypervisor's support for this interface and the location of the shared >>> memory structures. >>> >>> Signed-off-by: Steven Price <steven.price@xxxxxxx> >>> --- >>> Documentation/virt/kvm/arm/pvtime.txt | 100 ++++++++++++++++++++++++++ >>> 1 file changed, 100 insertions(+) >>> create mode 100644 Documentation/virt/kvm/arm/pvtime.txt >>> >>> diff --git a/Documentation/virt/kvm/arm/pvtime.txt b/Documentation/virt/kvm/arm/pvtime.txt >>> new file mode 100644 >>> index 000000000000..1ceb118694e7 >>> --- /dev/null >>> +++ b/Documentation/virt/kvm/arm/pvtime.txt >>> @@ -0,0 +1,100 @@ >>> +Paravirtualized time support for arm64 >>> +====================================== >>> + >>> +Arm specification DEN0057/A defined a standard for paravirtualised time >>> +support for AArch64 guests: >>> + >>> +https://developer.arm.com/docs/den0057/a >>> + >>> +KVM/arm64 implements the stolen time part of this specification by providing >>> +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. The existence of >>> +the PV_FEATURES hypercall should be probed using the SMCCC 1.1 ARCH_FEATURES >>> +mechanism before calling it. >>> + >>> +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. >>> + >>> +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) >>> + >>> +The IPA returned by PV_TIME_ST should be mapped by the guest as normal memory >>> +with inner and outer write back caching attributes, in the inner shareable >>> +domain. A total of 16 bytes from the IPA returned are guaranteed to be >>> +meaningfully filled by the hypervisor (see structure below). >>> + >>> +PV_TIME_ST returns the structure for the calling VCPU. >>> + >>> +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 prior to scheduling a 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 per VCPU of the guest. >>> + >>> +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); >>> + >>> +Creation of the device should be done after creating the vCPUs of the virtual >>> +machine. >>> + >>> +The IPA of the structures must be given to KVM. This is the base address >>> +of an array of stolen time structures (one for each VCPU). The base address >>> +must be page aligned. The size must be at least 64 * number of VCPUs and be a >>> +multiple of PAGE_SIZE. >>> + >>> +The memory for these structures should be added to the guest in the usual >>> +manner (e.g. using KVM_SET_USER_MEMORY_REGION). >>> + >>> +For example: >>> + >>> + struct kvm_dev_arm_st_region region = { >>> + .gpa = <IPA of guest base address>, >>> + .size = <size in bytes> >>> + }; >> >> This feel fragile; how are you handling userspace creating VCPUs after >> setting this up, the GPA overlapping guest memory, etc. Is the >> philosophy here that the VMM can mess up the VM if it wants, but that >> this should never lead attacks on the host (we better hope not) and so >> we don't care? >> >> It seems to me setting the IPA per vcpu throught the VCPU device would >> avoid a lot of these issues. See >> Documentation/virt/kvm/devices/vcpu.txt. >> >> > I discussed this with Marc the other day, and we realized that if we > make the configuration of the IPA per-PE, then a VMM can construct a VM > where these data structures are distributed within the IPA space of a > VM, which could lead to a lower TLB pressure for some > configurations/workloads. Ok, I'm dubious it will make much difference in terms of TLB pressure, but I've done the refactoring and I think it actually simplifies the code. So I'll post a new version where the base address is set via the VCPU device. Thanks for the review, Steve
