linux-kernel - Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR

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Date:   Fri, 10 Jul 2020 19:40:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Mohammed Gamal <mgamal@...hat.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR

On 10/07/20 19:26, Jim Mattson wrote:
>> Intel only has MAXPHYADDR > 46 on LA57 machines (because in general OSes
>> like to have a physical 1:1 map into the kernel part of the virtual
>> address space, so having a higher MAXPHYADDR would be of limited use
>> with 48-bit linear addresses).
> We all know that the direct map is evil. :-)
> 
> Sorry it took me so long to get there. I didn't realize that Linux was
> incapable of using more physical memory than it could map into the
> kernel's virtual address space. (Wasn't that the whole point of PAE
> originally?)

Yes, but it's so slow that Linux preferred not to go that way for 64-bit
kernels.

That said, that justification for MAXPHYADDR==46 came from Intel
processor architects, and when they say "OSes" they usually refer to a
certain vendor from the Pacific north-west.

Paolo