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

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Message-ID: <11618b8b-4d1f-9307-35f0-3c0f0fc856ca@amd.com>
Date:   Mon, 10 Sep 2018 10:30:39 -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 10:28 AM, Sean Christopherson wrote:
...

>>
>> 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 ?
> 
> KVM_MAX_VCPUS is a definition for use in the *host*, it's even defined
> in kvm_host.h.  The guest's pvclock code won't get magically recompiled
> if KVM_MAX_VCPUS is changed in the host.  KVM_MAX_VCPUS is an arbitrary
> value in the sense that there isn't a fundamental hard limit, i.e. the
> value can be changed, either for a custom KVM build or in mainline,
> e.g. it was bumped in 2016:
> 

Ah I see your point. thanks for clarifying it.