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From:   "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
Subject: Re: [PATCH v5.5 03/30] KVM: Require total number of memslot pages to
 fit in an unsigned long
To:     Sean Christopherson <seanjc@google.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
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        Wanpeng Li <wanpengli@tencent.com>,
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        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
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        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Ben Gardon <bgardon@google.com>, Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
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References: <20211104002531.1176691-1-seanjc@google.com>
 <20211104002531.1176691-4-seanjc@google.com>
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Date:   Tue, 9 Nov 2021 01:38:24 +0100
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On 04.11.2021 01:25, Sean Christopherson wrote:
> Explicitly disallow creating more memslot pages than can fit in an
> unsigned long, KVM doesn't correctly handle a total number of memslot
> pages that doesn't fit in an unsigned long and remedying that would be a
> waste of time.
> 
> For a 64-bit kernel, this is a nop as memslots are not allowed to overlap
> in the gfn address space.
> 
> With a 32-bit kernel, userspace can at most address 3gb of virtual memory,
> whereas wrapping the total number of pages would require 4tb+ of guest
> physical memory.  Even with x86's second address space for SMM, userspace
> would need to alias all of guest memory more than one _thousand_ times.
> And on older x86 hardware with MAXPHYADDR < 43, the guest couldn't
> actually access any of those aliases even if userspace lied about
> guest.MAXPHYADDR.
> 
> On 390 and arm64, this is a nop as they don't support 32-bit hosts.
> 
> On x86, practically speaking this is simply acknowledging reality as the
> existing kvm_mmu_calculate_default_mmu_pages() assumes the total number
> of pages fits in an "unsigned long".
> 
> On PPC, this is likely a nop as every flavor of PPC KVM assumes gfns (and
> gpas!) fit in unsigned long.  arch/powerpc/kvm/book3s_32_mmu_host.c goes
> a step further and fails the build if CONFIG_PTE_64BIT=y, which
> presumably means that it does't support 64-bit physical addresses.
> 
> On MIPS, this is also likely a nop as the core MMU helpers assume gpas
> fit in unsigned long, e.g. see kvm_mips_##name##_pte.
> 
> And finally, RISC-V is a "don't care" as it doesn't exist in any release,
> i.e. there is no established ABI to break.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>