Re: [PATCH v3 08/34] KVM: x86: hyper-v: Use HV_MAX_SPARSE_VCPU_BANKS/HV_VCPUS_PER_SPARSE_BANK instead of raw '64'

[Maxim Levitsky]

On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
> It may not be clear from where the '64' limit for the maximum sparse
> bank number comes from, use HV_MAX_SPARSE_VCPU_BANKS define instead.
> Use HV_VCPUS_PER_SPARSE_BANK in KVM_HV_MAX_SPARSE_VCPU_SET_BITS's
> definition. Opportunistically adjust the comment around BUILD_BUG_ON().
> 
> No functional change.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/kvm/hyperv.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index fc4bb0ead9fa..3cf68645a2e6 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -43,7 +43,7 @@
>  /* "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_VCPUS, HV_VCPUS_PER_SPARSE_BANK)
>  
>  static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
>  				bool vcpu_kick);
> @@ -1798,7 +1798,7 @@ static u64 kvm_get_sparse_vp_set(struct kvm *kvm, struct kvm_hv_hcall *hc,
>  				 u64 *sparse_banks, int consumed_xmm_halves,
>  				 gpa_t offset)
>  {
> -	if (hc->var_cnt > 64)
> +	if (hc->var_cnt > HV_MAX_SPARSE_VCPU_BANKS)
>  		return -EINVAL;
>  
>  	/* Cap var_cnt to ignore banks that cannot contain a legal VP index. */
> @@ -1969,12 +1969,11 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>  	gpa_t data_offset;
>  
>  	/*
> -	 * The Hyper-V TLFS doesn't allow more than 64 sparse banks, e.g. the
> -	 * valid mask is a u64.  Fail the build if KVM's max allowed number of
> -	 * vCPUs (>4096) would exceed this limit, KVM will additional changes
> -	 * for Hyper-V support to avoid setting the guest up to fail.
> +	 * The Hyper-V TLFS doesn't allow more than HV_MAX_SPARSE_VCPU_BANKS
> +	 * sparse banks. Fail the build if KVM's max allowed number of
> +	 * vCPUs (>4096) exceeds this limit.
>  	 */
> -	BUILD_BUG_ON(KVM_HV_MAX_SPARSE_VCPU_SET_BITS > 64);
> +	BUILD_BUG_ON(KVM_HV_MAX_SPARSE_VCPU_SET_BITS > HV_MAX_SPARSE_VCPU_BANKS);
>  
>  	if (!hc->fast && is_guest_mode(vcpu)) {
>  		hc->ingpa = translate_nested_gpa(vcpu, hc->ingpa, 0, NULL);

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky