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:   Tue, 17 May 2022 16:19:54 +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 Tue, May 17, 2022, Vitaly Kuznetsov wrote:
>> 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.
>
> Out of curiosity, is L1 "required" to provides the L2 => L1 translation/map?
>

To make this "Direct Virtual Flush" feature work? Yes, it is:

...
"
Before enabling it, the L1 hypervisor must configure the following
additional fields of the enlightened VMCS:
- VpId: ID of the virtual processor that the enlightened VMCS controls.
- VmId: ID of the virtual machine that the enlightened VMCS belongs to.
- PartitionAssistPage: Guest physical address of the partition assist
page.
"

-- 
Vitaly