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    SubjectRe: [PATCH v5 00/12] KVM: introduce readonly memslot
    On 08/14/2012 01:39 AM, Marcelo Tosatti wrote:
    > On Sat, Aug 11, 2012 at 11:36:20AM +0800, Xiao Guangrong wrote:
    >> On 08/11/2012 02:14 AM, Marcelo Tosatti wrote:
    >>> On Tue, Aug 07, 2012 at 05:47:15PM +0800, Xiao Guangrong wrote:
    >>>> Changelog:
    >>>> - introduce KVM_PFN_ERR_RO_FAULT instead of dummy page
    >>>> - introduce KVM_HVA_ERR_BAD and optimize error hva indicators
    >>>>
    >>>> The test case can be found at:
    >>>> http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/00819/migrate-perf.tar.bz2
    >>>>
    >>>> In current code, if we map a readonly memory space from host to guest
    >>>> and the page is not currently mapped in the host, we will get a fault-pfn
    >>>> and async is not allowed, then the vm will crash.
    >>>>
    >>>> As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD
    >>>> to the guest, read access is happy for readonly memslot, write access on
    >>>> readonly memslot will cause KVM_EXIT_MMIO exit.
    >>>
    >>> Memory slots whose QEMU mapping is write protected is supported
    >>> today, as long as there are no write faults.
    >>>
    >>> What prevents the use of mmap(!MAP_WRITE) to handle read-only memslots
    >>> again?
    >>>
    >>
    >> It is happy to map !write host memory space to the readonly memslot,
    >> and they can coexist as well.
    >>
    >> readonly memslot checks the write-permission by seeing slot->flags and
    >> !write memory checks the write-permission in hva_to_pfn() function
    >> which checks vma->flags. It is no conflict.
    >
    > Yes, there is no conflict. The point is, if you can use the
    > mmap(PROT_READ) interface (supporting read faults on read-only slots)
    > for this behavior, what is the advantage of a new memslot flag?
    >

    You can get the discussion at:
    https://lkml.org/lkml/2012/5/22/228

    > I'm not saying mmap(PROT_READ) is the best interface, i am just asking
    > why it is not.

    My fault. :(

    >
    >>> The initial objective was to fix a vm crash, can you explain that
    >>> initial problem?
    >>>
    >>
    >> The issue was trigged by this code:
    >>
    >> } else {
    >> if (async && (vma->vm_flags & VM_WRITE))
    >> *async = true;
    >> pfn = KVM_PFN_ERR_FAULT;
    >> }
    >>
    >> If the host memory region is readonly (!vma->vm_flags & VM_WRITE) and
    >> its physical page is swapped out (or the file data does not be read in),
    >> get_user_page_nowait will fail, above code reject to set async,
    >> then we will get a fault pfn and async=false.
    >>
    >> I guess this issue also exists in "QEMU write protected mapping" as
    >> you mentioned above.
    >
    > Yes, it does. As far as i understand, what that check does from a high
    > level pov is:
    >
    > - Did get_user_pages_nowait() fail due to a swapped out page (in which
    > case we should try to swappin the page asynchronously), or due to
    > another reason (for which case an error should be returned).
    >
    > Using vma->vm_flags VM_WRITE for that is trying to guess why
    > get_user_pages_nowait() failed, because it (gup_nowait return values)
    > does not provide sufficient information by itself.
    >

    That is exactly what i did in the first version. :)

    You can see it and the reason why it switched to the new way (readonly memslot)
    in the above website (the first message in thread).

    > Can't that be fixed separately?
    >
    > Another issue which is also present with the mmap(PROT_READ) scheme is
    > interaction with reexecute_instruction. That is, unless i am mistaken,
    > reexecute_instruction can succeed (return true) on a region that is
    > write protected. This breaks the "write faults on read-only slots exit
    > to userspace via EXIT_MMIO" behaviour.

    Sorry, Why? After re-entry to the guest, it can not generate a correct MMIO?




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