Re: [PATCH] kvm tools: adds a PCI device that exports a host shared segment as a PCI BAR in the guest

[Pekka Enberg <penberg@kernel.org>]

On 8/25/11 10:20 AM, Asias He wrote:
> On Thu, Aug 25, 2011 at 3:02 PM, Pekka Enberg<penberg@kernel.org>  wrote:
>> On 8/25/11 9:30 AM, Asias He wrote:
>>> On Thu, Aug 25, 2011 at 1:54 PM, Pekka Enberg<penberg@kernel.org>    wrote:
>>>> On 8/25/11 8:34 AM, Asias He wrote:
>>>>
>>>> Hi, David
>>>>
>>>> On Thu, Aug 25, 2011 at 6:25 AM, David Evensky<evensky@sandia.gov>
>>>>   wrote:
>>>>> This patch adds a PCI device that provides PCI device memory to the
>>>>> guest. This memory in the guest exists as a shared memory segment in
>>>>> the host. This is similar memory sharing capability of Nahanni
>>>>> (ivshmem) available in QEMU. In this case, the shared memory segment
>>>>> is exposed as a PCI BAR only.
>>>>>
>>>>> A new command line argument is added as:
>>>>>     --shmem pci:0xc8000000:16MB:handle=/newmem:create
>>>>>
>>>>> which will set the PCI BAR at 0xc8000000, the shared memory segment
>>>>> and the region pointed to by the BAR will be 16MB. On the host side
>>>>> the shm_open handle will be '/newmem', and the kvm tool will create
>>>>> the shared segment, set its size, and initialize it. If the size,
>>>>> handle, or create flag are absent, they will default to 16MB,
>>>>> handle=/kvm_shmem, and create will be false.
>>>> I think it's better to use a default BAR address if user does not specify
>>>> one as well.
>>>> This way,
>>>>
>>>> ./kvm --shmem
>>>>
>>>> will work with default values with zero configuration.
>>>>
>>>> Does that sort of thing make sense here? It's a special purpose device
>>>> and the guest is expected to ioremap() the memory so it needs to
>>>> know the BAR.
>>> I mean a default bar address for --shmem device.  Yes, guest needs to know
>>> this address, but even if we specify the address at command line the guest
>>> still
>>> does not know this address, no? So having a default bar address does no
>>> harm.
>> How does the user discover what the default BAR is? Which default BAR
>> should we use? I don't think default BAR adds much value here.
> 1. Print it on startup like, like we do for --name.
>    # kvm run -k ./bzImage -m 448 -c 4 --name guest-26676 --shmem bar=0xc8000000
>
> or
>
> 2. kvm stat --shmem
>
> David has chosen a default BAR already.
> #define SHMEM_DEFAULT_ADDR (0xc8000000)

OK. Makes sense.

                         Pekka