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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Message-ID: <87k0akuv1o.fsf@redhat.com>
Date:   Tue, 17 May 2022 15:51:15 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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'

Sean Christopherson <seanjc@...gle.com> writes:

> 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.

My brain is probably tainted by looking at all this for some time so I
really appreciate such improvements, thanks :)

I wouldn't, however, say "never" ('never say never' :-)): KVM could've
kept 2-level reverse mapping up-to-date:

KVM -> L2 VM list -> L2 vCPU ids -> L1 vCPUs which run them

making it possible for KVM to quickly translate between L2 VP IDs and L1
vCPUs. I don't do this in the series and just record L2 VM_ID/VP_ID for
each L1 vCPU so I have to go over them all for each request. The
optimization is, however, possible and we may get to it if really big
Windows VMs become a reality.

>
>> 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?
>

It certainly should, thanks!

-- 
Vitaly