linux-kernel - Re: [RFC 06/16] KVM: Use GUP instead of copy_from/to

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Message-ID: <a5acc4c8-2ee6-9e5d-c0a5-2a6f7c54c059@redhat.com>
Date:   Mon, 1 Jun 2020 18:35:22 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [RFC 06/16] KVM: Use GUP instead of copy_from/to_user() to access
 guest memory

On 25/05/20 17:17, Kirill A. Shutemov wrote:
>> Personally, I would've just added 'struct kvm' pointer to 'struct
>> kvm_memory_slot' to be able to extract 'mem_protected' info when
>> needed. This will make the patch much smaller.
> Okay, can do.
> 
> Other thing I tried is to have per-slot flag to indicate that it's
> protected. But Sean pointed that it's all-or-nothing feature and having
> the flag in the slot would be misleading.
> 

Perhaps it would be misleading, but it's an optimization.  Saving a
pointer dereference can be worth it, also because there are some places
where we just pass around a memslot and we don't have the struct kvm*.

Also, it's an all-or-nothing feature _now_.  It doesn't have to remain
that way.

Paolo