Re: [PATCH v3 3/4] x86/kvm: add max number of vcpus for hyperv emulation

[Sean Christopherson <seanjc@google.com>]

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.

>  #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
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.