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    SubjectRe: [PATCH v5 00/12] KVM: introduce readonly memslot
    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.

    > 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.



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