Re: [PATCH v5.5 03/30] KVM: Require total number of memslot pages to fit in an unsigned long

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 <[email protected]>

Reviewed-by: Maciej S. Szmigiero <[email protected]>