| From | "Maciej S. Szmigiero" <> | Subject | Re: [PATCH v5.5 03/30] KVM: Require total number of memslot pages to fit in an unsigned long | 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>
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