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Date: Wed, 12 Sep 2018 18:13:07 +0200
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Subject: Re: [Qemu-devel] [PATCH 0/8] Add missed read callback for some
 memory region
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To: Li Qiang <liq3ea@163.com>, pbonzini@redhat.com, lersek@redhat.com, marcandre.lureau@gmail.com, ehabkost@redhat.com, mst@redhat.com, peter.maydell@linaro.org, ppandit@redhat.com
Cc: Li Qiang <liq3ea@gmail.com>, qemu-devel@nongnu.org

Hi Li,

On 9/12/18 6:01 PM, Li Qiang wrote:
> From: Li Qiang <liq3ea@gmail.com>
> 
> This patch set try to add the missed read callback for memory region.
> Without this patchset, when the guest reads the IO port/memory, it will
> cause an NULL-dereference issue. For example, add 
> "-device isa-debug-exit" to command, then read the 0x501 port, it causes a 
> SIGSEGV.
> 
> The only exception is 'readonly_mem_ops' as its read is directly 
> access the underlying host ram as the comments says.
> 
> These missed read callback is mostly pointed by Laszlo Ersek.
> 
> 
> 
> Li Qiang (8):
>   fw_cfg_mem: add read memory region callback
>   hw: debugexit: add read callback
>   hw: hyperv_testdev: add read callback
>   hw: pc-testdev: add read memory region callback
>   hw: designware: add read memory region callback
>   hw: pvrdma: add read memory region callback
>   hw: sun4c: add read memory region callback
>   exec: add read callback for notdirty memory region

Why not rather simply add a check in
memory_region_oldmmio_read_accessor() instead?

Eventually:

{
    uint64_t tmp;
    int idx = ctz32(size);

    if (unlikely(mr->ops->old_mmio.write[idx]
                 && !mr->ops->old_mmio.read[idx])) {
        tmp = 0; /* XXX is 0 the expected value??? */
    } else {
        tmp = mr->ops->old_mmio.read[idx](mr->opaque, addr);
    }
    ...