Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support

[Ben Warren]

> On Feb 6, 2017, at 10:17 AM, Michael S. Tsirkin <address@hidden> wrote:
> 
> On Mon, Feb 06, 2017 at 09:59:55AM -0800, Ben Warren wrote:
>>               +void vmgenid_add_fw_cfg(FWCfgState *s, GArray *guid)
>>               +{
>>               +    Object *obj = find_vmgenid_dev(NULL);
>>               +    assert(obj);
>>               +    VmGenIdState *vms = VMGENID(obj);
>>               +
>>               +    /* Create a read-only fw_cfg file for GUID */
>>               +    fw_cfg_add_file(s, VMGENID_GUID_FW_CFG_FILE, guid->data,
>>               +                    VMGENID_FW_CFG_SIZE);
>>               +    /* Create a read-write fw_cfg file for Address */
>>               +    fw_cfg_add_file_callback(s, VMGENID_ADDR_FW_CFG_FILE, 
>> NULL,
>>        NULL,
>> 
>> 
>>           Seems wrong. What if guest updates the address after command line
>>           set it? You want a callback to copy guid there.
>> 
>> 
>>        Sure, I can do that.  My understanding was that this is a read
>>        callback, but if
>>        it also is called upon a write, we should do what you suggest.
>> 
>> 
>>    Hmm you are right. But we really need to call something
>>    on write though - unlike read, it must be called after write.
>>    Otherwise I don't see how it can work if you set gen id before
>>    guest boots.
>> 
>>    I guess this means we need yet another callback per file.
>>    FWCfgWriteCallback ?
>> 
>>    Can you implement this in hw/nvram/fw_cfg.c?
>>    It's rather straight-forward to do.
>> 
>> 
>> The reason it works is that we put the initial contents of the GUID (as
>> supplied by command-line) into the GUID fw_cfg in the ‘vmgenid_build_acpi()’
>> function, which is guaranteed to happen before the guest boots.  The only 
>> time
>> QEMU needs to know VGIA is on later updates to the GUID (via monitor) or when
>> restoring.  If you really think this extra complexity is needed, I can do so,
>> but it seems to work very well as-is.
> 
> I see. So it's a race condition I think.  It works unless you change the
> gen id after bios read the guid from the guid file but before it wrote
> out the address.
> 
> Or do I miss something?
> 
I don’t think it’s an issue because BIOS is not a consumer of the GUID.  I 
suppose there’s a window where another GUID could be written via monitor prior 
to bios writing back, but the change wouldn’t be applied to memory in 
vmgenid_update_guest() because vgia=0.  That would be solved by a write 
callback and we could remove the GUID copying from vmgenid_build_acpi().  It’s 
a very unlikely scenario, though.
> -- 
> MST

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