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Re: [Qemu-devel] Virtual Machine Generation ID

From: Ben Warren
Subject: Re: [Qemu-devel] Virtual Machine Generation ID
Date: Wed, 18 Jan 2017 16:02:31 -0800

Hi Michael,
> On Jan 17, 2017, at 9:45 AM, Michael S. Tsirkin <address@hidden> wrote:
> On Mon, Jan 16, 2017 at 10:57:42AM -0800, Ben Warren wrote:
>> I think we have a misunderstanding here.  I’m storing the VM
>> Generation ID __data__ (a GUID) in a fw_cfg blob, not the address.
> Yes, I think I gathered this much from the discussion. This is what
> I'm saying - don't. Have guest loader reserve guest memory and write the
> address into a fw cfg blob, and have qemu write the id at that address.
> This way you can update guest memory at any time.
So I’ve gone down the path of creating a writeable fw_cfg blob to hold the VGID 
address, but it doesn’t seem to be getting updated.

Here’s the code I’ve added:

#define VMGENID_FW_CFG_FILE      "etc/vmgenid"
#define VMGENID_FW_CFG_ADDR_FILE      "etc/vmgenid_addr”

// Create writeable fw_cfg blob, vas->vgia is a GArray of size 8 and element 
size 1
fw_cfg_add_file_callback(s, VMGENID_FW_CFG_ADDR_FILE, NULL, NULL, 
vms->vgia->data, 8, false);

// Request BIOS to allocate memory for the read-only DATA file:
bios_linker_loader_alloc(linker, VMGENID_FW_CFG_FILE, guid, 0,false);

// Request BIOS to allocate memory for the writeable ADDRESS file:
bios_linker_loader_alloc(linker, VMGENID_FW_CFG_ADDR_FILE, s->vgia, 0, false);

// Request BIOS to write the address of the DATA file into the ADDRESS file:
bios_linker_loader_add_pointer(linker, VMGENID_FW_CFG_ADDR_FILE, 0, 
sizeof(uint64_t), VMGENID_FW_CFG_FILE, 0);

I’ve instrumented SeaBIOS and see the requests being made and memcpy to the 
file happening, but don’t see any changes in QEMU in the memory pointed to by 
VMGENID_FW_CFG_ADDR_FILE.  Is this how writeable fw_cfg is supposed to work?


> -- 

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