From mboxrd@z Thu Jan  1 00:00:00 1970
From: Roman Kagan <rkagan@virtuozzo.com>
Subject: Re: [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx
 mapping
Date: Fri, 29 Jun 2018 13:11:36 +0300
Message-ID: <20180629101134.GA15656@rkaganb.sw.ru>
References: <20180628135313.17468-1-vkuznets@redhat.com>
 <20180628135313.17468-3-vkuznets@redhat.com>
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Cc: kvm@vger.kernel.org, x86@kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
        Mohammed Gamal <mmorsy@redhat.com>,
        Cathy Avery <cavery@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        linux-kernel@vger.kernel.org
To: Vitaly Kuznetsov <vkuznets@redhat.com>
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On Thu, Jun 28, 2018 at 03:53:10PM +0200, Vitaly Kuznetsov wrote:
> While it is easy to get VP index from vCPU index the reverse task is hard.
> Basically, to solve it we have to walk all vCPUs checking if their VP index
> matches. For hypercalls like HvFlushVirtualAddress{List,Space}* and the
> upcoming HvSendSyntheticClusterIpi* where a single CPU may be specified in
> the whole set this is obviously sub-optimal.
> 
> As VP index can be set to anything <= U32_MAX by userspace using plain
> [0..MAX_VP_INDEX] array is not a viable option. Use condensed sorted
> array with logarithmic search complexity instead. Use RCU to make read
> access as fast as possible and maintain atomicity of updates.

Quoting TLFS 5.0C section 7.8.1:

> Virtual processors are identified by using an index (VP index). The
> maximum number of virtual processors per partition supported by the
> current implementation of the hypervisor can be obtained through CPUID
> leaf 0x40000005. A virtual processor index must be less than the
> maximum number of virtual processors per partition.

so this is a dense index, and VP_INDEX >= KVM_MAX_VCPUS is invalid.  I
think we're better off enforcing this in kvm_hv_set_msr and keep the
translation simple.  If the algorithm in get_vcpu_by_vpidx is not good
enough (and yes it can be made to return NULL early on vpidx >=
KVM_MAX_VCPUS instead of taking the slow path) then a simple index array
of KVM_MAX_VCPUS entries should certainly do.

Roman.