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SubjectRe: [PATCH v3 3/4] x86/kvm: add max number of vcpus for hyperv emulation
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On 17.11.21 21:50, Sean Christopherson wrote:
> On Tue, Nov 16, 2021, Juergen Gross wrote:
>> When emulating Hyperv the theoretical maximum of vcpus supported is
>> 4096, as this is the architectural limit for sending IPIs via the PV
>> interface.
>>
>> For restricting the actual supported number of vcpus for that case
>> introduce another define KVM_MAX_HYPERV_VCPUS and set it to 1024, like
>> today's KVM_MAX_VCPUS. Make both values unsigned ones as this will be
>> needed later.
>>
>> The actual number of supported vcpus for Hyperv emulation will be the
>> lower value of both defines.
>>
>> This is a preparation for a future boot parameter support of the max
>> number of vcpus for a KVM guest.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V3:
>> - new patch
>> ---
>> arch/x86/include/asm/kvm_host.h | 3 ++-
>> arch/x86/kvm/hyperv.c | 15 ++++++++-------
>> 2 files changed, 10 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 886930ec8264..8ea03ff01c45 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -38,7 +38,8 @@
>>
>> #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
>>
>> -#define KVM_MAX_VCPUS 1024
>> +#define KVM_MAX_VCPUS 1024U
>> +#define KVM_MAX_HYPERV_VCPUS 1024U
>
> I don't see any reason to put this in kvm_host.h, it should never be used outside
> of hyperv.c.

Okay, fine with me.

>
>> #define KVM_MAX_VCPU_IDS kvm_max_vcpu_ids()
>> /* memory slots that are not exposed to userspace */
>> #define KVM_PRIVATE_MEM_SLOTS 3
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index 4a555f32885a..c0fa837121f1 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -41,7 +41,7 @@
>> /* "Hv#1" signature */
>> #define HYPERV_CPUID_SIGNATURE_EAX 0x31237648
>>
>> -#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
>> +#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_HYPERV_VCPUS, 64)
>>
>> static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
>> bool vcpu_kick);
>> @@ -166,7 +166,7 @@ static struct kvm_vcpu *get_vcpu_by_vpidx(struct kvm *kvm, u32 vpidx)
>> struct kvm_vcpu *vcpu = NULL;
>> int i;
>>
>> - if (vpidx >= KVM_MAX_VCPUS)
>> + if (vpidx >= min(KVM_MAX_VCPUS, KVM_MAX_HYPERV_VCPUS))
>
> IMO, this is conceptually wrong. KVM should refuse to allow Hyper-V to be enabled
> if the max number of vCPUs exceeds what can be supported, or should refuse to create

TBH, I wasn't sure where to put this test. Is there a guaranteed
sequence of ioctl()s regarding vcpu creation (or setting the max
number of vcpus) and the Hyper-V enabling?

> the vCPUs. I agree it makes sense to add a Hyper-V specific limit, since there are
> Hyper-V structures that have a hard limit, but detection of violations should be a
> BUILD_BUG_ON, not a silent failure at runtime.
>

A BUILD_BUG_ON won't be possible with KVM_MAX_VCPUS being selecteble via
boot parameter.


Juergen
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