linux-kernel - Re: [PATCH v3 11/34] KVM: x86: hyper-v: Use preallocated buffer in 'struct kvm_vcpu_hv' instead of on-stack 'sparse

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Date:   Mon, 16 May 2022 20:05:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 11/34] KVM: x86: hyper-v: Use preallocated buffer in
 'struct kvm_vcpu_hv' instead of on-stack 'sparse_banks'

On Thu, Apr 14, 2022, Vitaly Kuznetsov wrote:
> To make kvm_hv_flush_tlb() ready to handle L2 TLB flush requests, KVM needs
> to allow for all 64 sparse vCPU banks regardless of KVM_MAX_VCPUs as L1
> may use vCPU overcommit for L2. To avoid growing on-stack allocation, make
> 'sparse_banks' part of per-vCPU 'struct kvm_vcpu_hv' which is allocated
> dynamically.
> 
> Note: sparse_set_to_vcpu_mask() keeps using on-stack allocation as it
> won't be used to handle L2 TLB flush requests.

I think it's worth using stronger language; handling TLB flushes for L2 _can't_
use sparse_set_to_vcpu_mask() because KVM has no idea how to translate an L2
vCPU index to an L1 vCPU.  I found the above mildly confusing because it didn't
call out "vp_bitmap" and so I assumed the note referred to yet another sparse_banks
"allocation".  And while vp_bitmap is related to sparse_banks, it tracks something
entirely different.

Something like?

Note: sparse_set_to_vcpu_mask() can never be used to handle L2 requests as
KVM can't translate L2 vCPU indices to L1 vCPUs, i.e. its vp_bitmap array
is still bounded by the number of L1 vCPUs and so can remain an on-stack
allocation.

> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 3 +++
>  arch/x86/kvm/hyperv.c           | 6 ++++--
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 058061621872..837c07e213de 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -619,6 +619,9 @@ struct kvm_vcpu_hv {
>  	} cpuid_cache;
>  
>  	struct kvm_vcpu_hv_tlb_flush_ring tlb_flush_ring[HV_NR_TLB_FLUSH_RINGS];
> +
> +	/* Preallocated buffer for handling hypercalls passing sparse vCPU set */
> +	u64 sparse_banks[64];

Shouldn't this be HV_MAX_SPARSE_VCPU_BANKS?