linux-kernel - Re: [RFC PATCH 03/13] kvm: Add XO memslot type

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Date:   Sun, 6 Oct 2019 18:15:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Cc:     "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
        "Dock, Deneen T" <deneen.t.dock@...el.com>,
        "yu.c.zhang@...ux.intel.com" <yu.c.zhang@...ux.intel.com>
Subject: Re: [RFC PATCH 03/13] kvm: Add XO memslot type

On 04/10/19 21:06, Edgecombe, Rick P wrote:
> The reasoning was that it seems like KVM leaves it to userspace to control the
> physical address space layout since userspace decides the supported physical
> address bits and lays out memory in the physical address space. So duplication
> with XO memslots was an attempt was to keep the logic around that together.
> 
> I'll take another look at doing it this way though. I think userspace may still
> need to adjust the MAXPHYADDR and be aware it can't layout memory in the XO
> range.

Right, you would have to use KVM_ENABLE_CAP passing the desired X bit
(which must be < MAXPHYADDR) as the argument.  Userspace needs to know
that it must then make MAXPHYADDR in the guest CPUID equal to the
argument.  When the MSR is written to 1, bit "MAXPHYADDR-1" in the page
table entries becomes an XO bit.

Paolo