linux-kernel - Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

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Date:   Mon, 10 Sep 2018 10:10:09 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Borislav Petkov <bp@...e.de>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data
 when SEV is active



On 09/10/2018 08:29 AM, Sean Christopherson wrote:
...

>>>> + */
>>>> +static struct pvclock_vsyscall_time_info
>>>> +			hv_clock_aux[NR_CPUS] __decrypted_aux;
>>> Hmm, so worst case that's 64 4K pages:
>>>
>>> (8192*32)/4096 = 64 4K pages.
>> We can minimize the worst case memory usage. The number of VCPUs
>> supported by KVM maybe less than NR_CPUS. e.g Currently KVM_MAX_VCPUS is
>> set to 288
> 
> KVM_MAX_VCPUS is a property of the host, whereas this code runs in the
> guest, e.g. KVM_MAX_VCPUS could be 2048 in the host for all we know.
> 


IIRC, during guest creation time qemu will check the host supported
VCPUS count. If count is greater than KVM_MAX_VCPUS then it will
fail to launch guest (or fail to hot plug vcpus). In other words, the
number of vcpus in a KVM guest will never to > KVM_MAX_VCPUS.

Am I missing something ?