linux-kernel - Re: [RFC PATCH] kvm: x86/vmx: Use kzalloc for cached

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Date:   Tue, 15 Jan 2019 11:15:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Tom Roeder <tmroeder@...gle.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com
Subject: Re: [RFC PATCH] kvm: x86/vmx: Use kzalloc for cached_vmcs12

On 15/01/19 03:43, Sean Christopherson wrote:
>> -	vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
>> +	vmx->nested.cached_vmcs12 = kzalloc(VMCS12_SIZE, GFP_KERNEL);
>>  	if (!vmx->nested.cached_vmcs12)
>>  		goto out_cached_vmcs12;
> Obviously not your code, but why do we allocate VMCS12_SIZE instead of
> sizeof(struct vmcs12)?  I get why we require userspace to reserve the
> full 4k, but I don't understand why KVM needs to allocate the reserved
> bytes internally.

It's just cleaner and shorter code to copy everything in and out,
instead of having to explicitly zero the slack.

Paolo