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SubjectRe: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization
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On 9/11/2021 7:55 AM, Sean Christopherson wrote:
> On Fri, Sep 10, 2021, Sean Christopherson wrote:
>> On Mon, Aug 09, 2021, Zeng Guang wrote:
>>> + if (!pages)
>>> + return -ENOMEM;
>>> +
>>> + to_kvm_vmx(kvm)->pid_table = (void *)page_address(pages);
>>> + to_kvm_vmx(kvm)->pid_last_index = KVM_MAX_VCPU_ID;
>> I don't see the point of pid_last_index if we're hardcoding it to KVM_MAX_VCPU_ID.
>> If I understand the ucode pseudocode, there's no performance hit in the happy
>> case, i.e. it only guards against out-of-bounds accesses.
>>
>> And I wonder if we want to fail the build if this grows beyond an order-1
>> allocation, e.g.
>>
>> BUILD_BUG_ON(PID_TABLE_ORDER > 1);
>>
>> Allocating two pages per VM isn't terrible, but 4+ starts to get painful when
>> considering the fact that most VMs aren't going to need more than one page. For
>> now I agree the simplicity of not dynamically growing the table is worth burning
>> a page.
> Ugh, Paolo has queued a series which bumps KVM_MAX_VCPU_ID to 4096[*]. That makes
> this an order-3 allocation, which is quite painful. One thought would be to let
> userspace declare the max vCPU it wants to create, not sure if that would work for
> xAPIC though.
>
> [*] https://lkml.kernel.org/r/1111efc8-b32f-bd50-2c0f-4c6f506b544b@redhat.com
Thus we keep current design as no change.

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