Re: [RFC PATCH v6 14/35] KVM: arm64: Add SPE VCPU device attribute to set the max buffer size

James Clark
2026-01-12T11:50:28Z

On 12/01/2026 11:28 am, Alexandru Elisei wrote:
> Hi James,
>
> On Fri, Jan 09, 2026 at 04:29:43PM +0000, James Clark wrote:
>>
>>
>> On 14/11/2025 4:06 pm, Alexandru Elisei wrote:
>>> During profiling, the buffer programmed by the guest must be kept mapped at
>>> stage 2 by KVM, making this memory pinned from the host's perspective.
>>>
>>> To make sure that a guest doesn't consume too much memory, add a new SPE
>>> VCPU device attribute, KVM_ARM_VCPU_MAX_BUFFER_SIZE, which is used by
>>> userspace to limit the amount of memory a VCPU can pin when programming
>>> the profiling buffer. This value will be advertised to the guest in the
>>> PMBIDR_EL1.MaxBuffSize field.
>>>
>>> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
>>> ---
>>> Documentation/virt/kvm/devices/vcpu.rst | 49 ++++++++++
>>> arch/arm64/include/asm/kvm_spe.h | 6 ++
>>> arch/arm64/include/uapi/asm/kvm.h | 5 +-
>>> arch/arm64/kvm/arm.c | 2 +
>>> arch/arm64/kvm/spe.c | 116 ++++++++++++++++++++++++
>>> 5 files changed, 176 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
>>> index e305377fadad..bb1bbd2ff6e2 100644
>>> --- a/Documentation/virt/kvm/devices/vcpu.rst
>>> +++ b/Documentation/virt/kvm/devices/vcpu.rst
>>> @@ -347,3 +347,52 @@ attempting to set a different one will result in an error.
>>> Similar to KVM_ARM_VCPU_PMU_V3_CTRL(KVM_ARM_VCPU_PMU_SET_PMU), userspace is
>>> responsible for making sure that the VCPU is run only on physical CPUs which
>>> have the specified SPU.
>>> +
>>> +5.3 ATTRIBUTE: KVM_ARM_VCPU_MAX_BUFFER_SIZE
>>> +------------------------------------------
>>> +
>>> +:Parameters: in kvm_device_attr.addr the address to an u64 representing the
>>> + maximum buffer size, in bytes.
>>> +
>>> +:Returns:
>>> +
>>> + ======= =========================================================
>>> + -EBUSY Virtual machine has already run
>>> + -EDOM Buffer size cannot be represented by hardware
>>> + -EFAULT Error accessing the max buffer size identifier
>>> + -EINVAL A different maximum buffer size already set or the size is
>>> + not aligned to the host's page size
>>> + -ENXIO SPE not supported or not properly configured
>>> + -ENODEV KVM_ARM_VCPU_HAS_SPE VCPU feature or SPU instance not set
>>
>> Hi Alex,
>>
>> I can't reproduce this anymore, but I got this a few times. Or at least I
>> think it was this, I've pasted the output from kvmtool below and it doesn't
>> say exactly what the issue was.
>
> I'll try to reproduce it.
>
> Do you remember what were the HEAD commits for the host and kvmtool?
>

I was testing on N1SDP with the SPE driver changes as well so it has a
load of junk, although I don't think any could have caused this
intermittent issue. I wouldn't put too much effort into it though
because it could have been a stale build or something:

https://gitlab.com/Linaro/kwg/james-c-linux/-/tree/james-testing-kvm-spe-v6

kvmtool is just 8890373d5e62 from your kvm-spe-v6 branch

>>
>> If I tried again with a different buffer size it worked, then going back to
>> 256M didn't work, then it went away. I might have done something wrong so if
>> you didn't see this either then we can probably ignore it for now.
>>
>> -> sudo lkvm run --kernel /boot/vmlinux-6.18.0-rc2+ -p "earlycon
>> kpti=off" -c 4 -m 2000 --pmu --spe --spe-max-buffer-size=256M
>>
>> Info: # lkvm run -k /boot/vmlinux-6.18.0-rc2+ -m 2000 -c 4 --name
>> guest-616
>> KVM_SET_DEVICE_ATTR: No such device or address
>>
>>
>>> + -ERANGE Buffer size larger than maximum supported by the SPU
>>> + instance.
>>> + ======= ==========================================================
>>> +
>>> +Required.
>>> +
>>> +Limit the size of the profiling buffer for the VCPU to the specified value. The
>>> +value will be used by all VCPUs. Can be set for more than one VCPUs, as long as
>>> +the value stays the same.
>>> +
>>> +Requires that a SPU has been already assigned to the VM. The maximum buffer size
>>
>> Very minor nit, but would "Initialised with SPE" be better? Because it's
>> done through KVM_ARM_VCPU_INIT rather than "ASSIGN_SPU". I think it might
>> make it easier to understand how you are supposed to use it.
>>
>> SPU is never expanded either and I think users probably wouldn't be familiar
>> with what that is. A lot of times we could just say "has SPE" and it would
>> be clearer. I don't think separating the concepts of SPE and SPU gives us
>> anything in this high level of a doc other than potentially confusing users.
>
> Sure.
>
>>
>>> +must be less than or equal to the maximum buffer size of the assigned SPU instance,
>>
>> I don't understand this part. Do you mean "of the assigned physical SPU
>> instance"? The ARM states "no limit" is the only valid value here:
>
> Yes, physical instance.
>
>>
>> Reads as 0x0000
>> The only permitted value is 0x0000, indicating there is no limit to
>> the maximum buffer size.
>>
>> It would be good to expand on where the limit you are talking about comes
>> from.
>
> The hardware value might change in the future. Or the host might be running
> under nested virtualization, which makes having a different value likely. Like
> you said above, I don't think it's necessary to get into this much detail here -
> the idea I was trying to convey is that userspace cannot set the maximum buffer
> size to a value larger than what the physical SPU instance supports.
>

Ok makes sense, thanks.

>>
>>> +unless there is no limit on the maximum buffer size for the SPU. In this case
>>> +the VCPU maximum buffer size can have any value, including 0, as long as it can
>>> +be encoded by hardware. For details on how the hardware encodes this value,
>>> +please consult Arm DDI0601 for the field PMBIDR_EL1.MaxBuffSize.
>>> +
>>> +The value 0 is special and it means that there is no upper limit on the size of
>>> +the buffer that the guest can use. Can only be set if the SPU instance used by
>>> +the VM has a similarly unlimited buffer size.
>>
>> This is a comment about changes in kvmtool, but it's semi related so I'll
>> leave it here. But you say only half of the buffer is used at a time:
>>
>> In a guest, perf, when the user is root, uses the default value of 4MB
>> for the total size of the profiling memory. This is split in two by
>> the SPE driver, and at any given time only one half (2MB) is
>> programmed for the SPE buffer.
>>
>> However, KVM also has to pin the stage 1 translation tables that
>> translate the buffer, so if the default were 2MB, KVM would definitely
>> exceed this value. Make the default 4MB to avoid potential errors when
>> the limit is exceeded.
>>
>> But isn't that just for snapshot mode? In normal mode the half way point is
>> set to perf_output_handle->wakeup which comes from the watermark set by
>> userspace? If you set it to the end then in theory the whole buffer could be
>> used?
>
> Sure, I'll change the comment to say that 4MiB was chosen because that was the
> default in perf, and not go into more details.
>
> Thanks,
> Alex
>

I don't know if kvmtool is going to get used in production, or if anyone
is going to copy this default value though? If that might happen then
maybe a bigger value is better in case there is some tool or script that
has a different watermark setting and it doesn't work.

I think we can assume that if someone is enabling SPE then they're not
memory constrained and we don't need to worry about saving a few MB.

>>
>>> +
>>> +When a guest enables SPE on the VCPU, KVM will pin the host memory backing the
>>> +buffer to avoid the statistical profiling unit experiencing stage 2 faults when
>>> +it writes to memory. This includes the host pages backing the guest's stage 1
>>> +translation tables that are used to translate the buffer. As a result, it is
>>> +expected that the size of the memory that will be pinned for each VCPU will be
>>> +slightly larger that the maximum buffer set with this ioctl.
>>> +
>>> +This memory that is pinned will count towards the process RLIMIT_MEMLOCK. To
>>> +avoid the limit being exceeded, userspace must increase the RLIMIT_MEMLOCK limit
>>> +prior to running the VCPU, otherwise KVM_RUN will return to userspace with an
>>> +error.
>>> diff --git a/arch/arm64/include/asm/kvm_spe.h b/arch/arm64/include/asm/kvm_spe.h
>>> index a4e9f03e3751..e48f7a7f67bb 100644
>>> --- a/arch/arm64/include/asm/kvm_spe.h
>>> +++ b/arch/arm64/include/asm/kvm_spe.h
>>> @@ -12,6 +12,7 @@
>>> struct kvm_spe {
>>> struct arm_spe_pmu *arm_spu;
>>> + u64 max_buffer_size; /* Maximum per VCPU buffer size */
>>> };
>>> struct kvm_vcpu_spe {
>>> @@ -28,6 +29,8 @@ static __always_inline bool kvm_supports_spe(void)
>>> #define vcpu_has_spe(vcpu) \
>>> (vcpu_has_feature(vcpu, KVM_ARM_VCPU_SPE))
>>> +void kvm_spe_init_vm(struct kvm *kvm);
>>> +
>>> int kvm_spe_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr);
>>> int kvm_spe_get_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr);
>>> int kvm_spe_has_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr);
>>> @@ -41,6 +44,9 @@ struct kvm_vcpu_spe {
>>> #define kvm_supports_spe() false
>>> #define vcpu_has_spe(vcpu) false
>>> +static inline void kvm_spe_init_vm(struct kvm *kvm)
>>> +{
>>> +}
>>> static inline int kvm_spe_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
>>> {
>>> return -ENXIO;
>>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
>>> index 760c3e074d3d..9db652392781 100644
>>> --- a/arch/arm64/include/uapi/asm/kvm.h
>>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>>> @@ -445,8 +445,9 @@ enum {
>>> #define KVM_ARM_VCPU_PVTIME_CTRL 2
>>> #define KVM_ARM_VCPU_PVTIME_IPA 0
>>> #define KVM_ARM_VCPU_SPE_CTRL 3
>>> -#define KVM_ARM_VCPU_SPE_IRQ 0
>>> -#define KVM_ARM_VCPU_SPE_SPU 1
>>> +#define KVM_ARM_VCPU_SPE_IRQ 0
>>> +#define KVM_ARM_VCPU_SPE_SPU 1
>>> +#define KVM_ARM_VCPU_SPE_MAX_BUFFER_SIZE 2
>>> /* KVM_IRQ_LINE irq field index values */
>>> #define KVM_ARM_IRQ_VCPU2_SHIFT 28
>>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>>> index d7f802035970..9afdf66be8b2 100644
>>> --- a/arch/arm64/kvm/arm.c
>>> +++ b/arch/arm64/kvm/arm.c
>>> @@ -194,6 +194,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>> kvm_timer_init_vm(kvm);
>>> + kvm_spe_init_vm(kvm);
>>> +
>>> /* The maximum number of VCPUs is limited by the host's GIC model */
>>> kvm->max_vcpus = kvm_arm_default_max_vcpus();
>>> diff --git a/arch/arm64/kvm/spe.c b/arch/arm64/kvm/spe.c
>>> index c581838029ae..3478da2a1f7c 100644
>>> --- a/arch/arm64/kvm/spe.c
>>> +++ b/arch/arm64/kvm/spe.c
>>> @@ -3,6 +3,7 @@
>>> * Copyright (C) 2021 - ARM Ltd
>>> */
>>> +#include <linux/bitops.h>
>>> #include <linux/cpumask.h>
>>> #include <linux/kvm_host.h>
>>> #include <linux/perf/arm_spe_pmu.h>
>>> @@ -41,6 +42,99 @@ void kvm_host_spe_init(struct arm_spe_pmu *arm_spu)
>>> static_branch_enable(&kvm_spe_available);
>>> }
>>> +/*
>>> + * The maximum buffer size can be zero (no restrictions on the buffer size), so
>>> + * this value cannot be used as the uninitialized value. The maximum buffer size
>>> + * must be page aligned, so arbitrarily choose the value '1' for an
>>> + * uninitialized maximum buffer size.
>>> + */
>>> +#define KVM_SPE_MAX_BUFFER_SIZE_UNSET 1
>>> +
>>> +void kvm_spe_init_vm(struct kvm *kvm)
>>> +{
>>> + kvm->arch.kvm_spe.max_buffer_size = KVM_SPE_MAX_BUFFER_SIZE_UNSET;
>>> +}
>>> +
>>> +static u64 max_buffer_size_to_pmbidr_el1(u64 size)
>>> +{
>>> + u64 msb_idx, num_bits;
>>> + u64 maxbuffsize;
>>> + u64 m, e;
>>> +
>>> + /*
>>> + * size = m:zeros(12); m is 9 bits.
>>> + */
>>> + if (size <= GENMASK_ULL(20, 12)) {
>>> + m = size >> 12;
>>> + e = 0;
>>> + goto out;
>>> + }
>>> +
>>> + /*
>>> + * size = 1:m:zeros(e+11)
>>> + */
>>> +
>>> + num_bits = fls64(size);
>>> + msb_idx = num_bits - 1;
>>> +
>>> + /* MSB is not encoded. */
>>> + m = size & ~BIT(msb_idx);
>>> + /* m is 9 bits. */
>>> + m >>= msb_idx - 9;
>>> + /* MSB is not encoded, m is 9 bits wide and 11 bits are zero. */
>>> + e = num_bits - 1 - 9 - 11;
>>> +
>>> +out:
>>> + maxbuffsize = FIELD_PREP(GENMASK_ULL(8, 0), m) | \
>>> + FIELD_PREP(GENMASK_ULL(13, 9), e);
>>> + return FIELD_PREP(PMBIDR_EL1_MaxBuffSize, maxbuffsize);
>>> +}
>>> +
>>> +static u64 pmbidr_el1_to_max_buffer_size(u64 pmbidr_el1)
>>> +{
>>> + u64 maxbuffsize;
>>> + u64 e, m;
>>> +
>>> + maxbuffsize = FIELD_GET(PMBIDR_EL1_MaxBuffSize, pmbidr_el1);
>>> + e = FIELD_GET(GENMASK_ULL(13, 9), maxbuffsize);
>>> + m = FIELD_GET(GENMASK_ULL(8, 0), maxbuffsize);
>>> +
>>> + if (!e)
>>> + return m << 12;
>>> + return (1ULL << (9 + e + 11)) | (m << (e + 11));
>>> +}
>>> +
>>> +static int kvm_spe_set_max_buffer_size(struct kvm_vcpu *vcpu, u64 size)
>>> +{
>>> + struct kvm *kvm = vcpu->kvm;
>>> + struct kvm_spe *kvm_spe = &kvm->arch.kvm_spe;
>>> + u64 decoded_size, spu_size;
>>> +
>>> + if (kvm_vm_has_ran_once(kvm))
>>> + return -EBUSY;
>>> +
>>> + if (!PAGE_ALIGNED(size))
>>> + return -EINVAL;
>>> +
>>> + if (!kvm_spe->arm_spu)
>>> + return -ENODEV;
>>> +
>>> + if (kvm_spe->max_buffer_size != KVM_SPE_MAX_BUFFER_SIZE_UNSET)
>>> + return size == kvm_spe->max_buffer_size ? 0 : -EINVAL;
>>> +
>>> + decoded_size = pmbidr_el1_to_max_buffer_size(max_buffer_size_to_pmbidr_el1(size));
>>> + if (decoded_size != size)
>>> + return -EDOM;
>>> +
>>> + spu_size = pmbidr_el1_to_max_buffer_size(kvm_spe->arm_spu->pmbidr_el1);
>>> + if (spu_size != 0 && (size == 0 || size > spu_size))
>>> + return -ERANGE;
>>> +
>>> + kvm_spe->max_buffer_size = size;
>>> +
>>> + return 0;
>>> +}
>>> +
>>> static int kvm_spe_set_spu(struct kvm_vcpu *vcpu, int spu_id)
>>> {
>>> struct kvm *kvm = vcpu->kvm;
>>> @@ -136,6 +230,15 @@ int kvm_spe_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
>>> return kvm_spe_set_spu(vcpu, spu_id);
>>> }
>>> + case KVM_ARM_VCPU_SPE_MAX_BUFFER_SIZE: {
>>> + u64 __user *uaddr = (u64 __user *)(long)attr->addr;
>>> + u64 size;
>>> +
>>> + if (get_user(size, uaddr))
>>> + return -EFAULT;
>>> +
>>> + return kvm_spe_set_max_buffer_size(vcpu, size);
>>> + }
>>> }
>>> return -ENXIO;
>>> @@ -181,6 +284,18 @@ int kvm_spe_get_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
>>> return 0;
>>> }
>>> + case KVM_ARM_VCPU_SPE_MAX_BUFFER_SIZE: {
>>> + u64 __user *uaddr = (u64 __user *)(long)attr->addr;
>>> + u64 size = kvm_spe->max_buffer_size;
>>> +
>>> + if (size == KVM_SPE_MAX_BUFFER_SIZE_UNSET)
>>> + return -EINVAL;
>>> +
>>> + if (put_user(size, uaddr))
>>> + return -EFAULT;
>>> +
>>> + return 0;
>>> + }
>>> }
>>> return -ENXIO;
>>> @@ -194,6 +309,7 @@ int kvm_spe_has_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
>>> switch(attr->attr) {
>>> case KVM_ARM_VCPU_SPE_IRQ:
>>> case KVM_ARM_VCPU_SPE_SPU:
>>> + case KVM_ARM_VCPU_SPE_MAX_BUFFER_SIZE:
>>> return 0;
>>> }
>>