linux-kernel - Re: [PATCH v3 2/6] KVM: X86: Implement PV IPIs in linux guest

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Message-ID: <2a7a78b3-cc4e-de17-ee5d-6f6582683d34@redhat.com>
Date:   Fri, 20 Jul 2018 10:06:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v3 2/6] KVM: X86: Implement PV IPIs in linux guest

On 20/07/2018 07:58, Wanpeng Li wrote:
>>
>> We could keep the cluster size of 128, but it would be more complicated
>> to do the left shift in the first "else if".  If the limit is 64, you
>> can keep the two arguments in the hypercall, and just pass 0 as the
>> "high" bitmap on 64-bit kernels.
> As David pointed out, we need to scale to higher APIC IDs.

The offset is enough to scale to higher APIC IDs.

It's just an optimization to allow 128 CPUs per hypercall instead of 64
CPUs.  But actually you can use __uint128_t on 64-bit machines, I forgot
about that.  With u64 on 32-bit and __uint128_t on 64-bit, you can do 64
CPUs per hypercall on 32-bit and 128 CPUs per hypercall on 64-bit.

 I will add
> the cpu id to apic id transfer in the for loop. How about:
> kvm_hypercall2(KVM_HC_SEND_IPI, ipi_bitmap, vector); directly. In
> addition, why need to pass the 0 as the "high" bitmap even if for 128
> vCPUs case?